Are you using the same Qt to build against and to run? If you're unsure, the following commands might help to debug this:
rpm -q qt-devel
ldd /opt/kvirc/bin/kvirc4
ld --verbose -lQtGui -lQtCore ; rm -f a.out

A log from Valgrind would probably also be helpful. Something appears to corrupt the QFont's reference count, so copying the font crashes. Or it may be a use-after-free. Both are classes of bugs Valgrind should help figuring out. So can you please run:
valgrind /opt/kvirc/bin/kvirc4 2>kvirc4-valgrind.log
and attach the resulting kvirc4-valgrind.log file?

I don't remember having Qt installed myself.


muelli@bigbox ~/svn/kvirc $ rpm -q qt-devel
qt-devel-4.5.2-3.fc10.x86_64
muelli@bigbox ~/svn/kvirc $ ldd /opt/kvirc/bin/kvirc4
	linux-vdso.so.1 =>  (0x00007fffa44fe000)
	libperl.so => not found
	libresolv.so.2 => /lib64/libresolv.so.2 (0x0000003fbee00000)
	libnsl.so.1 => /lib64/libnsl.so.1 (0x0000003fbf600000)
	libdl.so.2 => /lib64/libdl.so.2 (0x0000003fb4c00000)
	libcrypt.so.1 => /lib64/libcrypt.so.1 (0x0000003fc3600000)
	libutil.so.1 => /lib64/libutil.so.1 (0x0000003fb3400000)
	libpthread.so.0 => /lib64/libpthread.so.0 (0x0000003fb5000000)
	libm.so.6 => /lib64/libm.so.6 (0x0000003fb4800000)
	libpython2.5.so.1.0 => /usr/lib64/libpython2.5.so.1.0 (0x0000003fcd600000)
	libkvilib4.so.4 => not found
	libz.so.1 => /lib64/libz.so.1 (0x0000003fb5400000)
	libQtWebKit.so.4 => /usr/lib64/libQtWebKit.so.4 (0x00000037f7600000)
	libQtGui.so.4 => /usr/lib64/libQtGui.so.4 (0x00000037f3200000)
	libpng12.so.0 => /usr/lib64/libpng12.so.0 (0x00000034c6800000)
	libSM.so.6 => /usr/lib64/libSM.so.6 (0x0000003e23600000)
	libICE.so.6 => /usr/lib64/libICE.so.6 (0x0000003fbfe00000)
	libXi.so.6 => /usr/lib64/libXi.so.6 (0x000000333b400000)
	libXrender.so.1 => /usr/lib64/libXrender.so.1 (0x000000333ac00000)
	libXrandr.so.2 => /usr/lib64/libXrandr.so.2 (0x000000333c000000)
	libXcursor.so.1 => /usr/lib64/libXcursor.so.1 (0x000000333bc00000)
	libXinerama.so.1 => /usr/lib64/libXinerama.so.1 (0x000000333b000000)
	libXfixes.so.3 => /usr/lib64/libXfixes.so.3 (0x000000333b800000)
	libfreetype.so.6 => /usr/lib64/libfreetype.so.6 (0x00000030a6a00000)
	libfontconfig.so.1 => /usr/lib64/libfontconfig.so.1 (0x00000030a6e00000)
	libXext.so.6 => /usr/lib64/libXext.so.6 (0x000000333a800000)
	libX11.so.6 => /usr/lib64/libX11.so.6 (0x0000003339c00000)
	libQtDBus.so.4 => /usr/lib64/libQtDBus.so.4 (0x00000037f2e00000)
	libQtXml.so.4 => /usr/lib64/libQtXml.so.4 (0x00000037f2200000)
	libQtSql.so.4 => /usr/lib64/libQtSql.so.4 (0x00000037f5a00000)
	libQtNetwork.so.4 => /usr/lib64/libQtNetwork.so.4 (0x00000037f2a00000)
	libssl.so.7 => /lib64/libssl.so.7 (0x0000003e24200000)
	libQtCore.so.4 => /usr/lib64/libQtCore.so.4 (0x00000037f1c00000)
	libgthread-2.0.so.0 => /lib64/libgthread-2.0.so.0 (0x0000003d2b600000)
	libglib-2.0.so.0 => /lib64/libglib-2.0.so.0 (0x0000003d2ae00000)
	librt.so.1 => /lib64/librt.so.1 (0x0000003fb6000000)
	libesd.so.0 => /usr/lib64/libesd.so.0 (0x0000003691800000)
	libaudiofile.so.0 => /usr/lib64/libaudiofile.so.0 (0x0000003fc3a00000)
	libcrypto.so.7 => /lib64/libcrypto.so.7 (0x00000034c8200000)
	libc.so.6 => /lib64/libc.so.6 (0x0000003fb4400000)
	libstdc++.so.6 => /usr/lib64/libstdc++.so.6 (0x0000003fbfa00000)
	libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x0000003fbe600000)
	/lib64/ld-linux-x86-64.so.2 (0x0000003fb3000000)
	libsqlite3.so.0 => /usr/lib64/libsqlite3.so.0 (0x0000003251400000)
	libphonon.so.4 => /usr/lib64/libphonon.so.4 (0x00000037f6600000)
	libdbus-1.so.3 => /lib64/libdbus-1.so.3 (0x000000333de00000)
	libgobject-2.0.so.0 => /lib64/libgobject-2.0.so.0 (0x0000003d2b200000)
	libuuid.so.1 => /lib64/libuuid.so.1 (0x0000003e22e00000)
	libexpat.so.1 => /lib64/libexpat.so.1 (0x0000003fb8000000)
	libXau.so.6 => /usr/lib64/libXau.so.6 (0x0000003fb6c00000)
	libxcb-xlib.so.0 => /usr/lib64/libxcb-xlib.so.0 (0x000000333a000000)
	libxcb.so.1 => /usr/lib64/libxcb.so.1 (0x000000333a400000)
	libgssapi_krb5.so.2 => /usr/lib64/libgssapi_krb5.so.2 (0x0000003e23a00000)
	libkrb5.so.3 => /usr/lib64/libkrb5.so.3 (0x0000003e23e00000)
	libcom_err.so.2 => /lib64/libcom_err.so.2 (0x0000003e23200000)
	libk5crypto.so.3 => /usr/lib64/libk5crypto.so.3 (0x0000003508800000)
	libasound.so.2 => /lib64/libasound.so.2 (0x0000003691400000)
	libcap.so.2 => /lib64/libcap.so.2 (0x0000003fbd600000)
	libXdmcp.so.6 => /usr/lib64/libXdmcp.so.6 (0x0000003fb7400000)
	libkrb5support.so.0 => /usr/lib64/libkrb5support.so.0 (0x0000003508400000)
	libkeyutils.so.1 => /lib64/libkeyutils.so.1 (0x0000003fc0a00000)
	libselinux.so.1 => /lib64/libselinux.so.1 (0x0000003840600000)
muelli@bigbox ~/svn/kvirc $ ld --verbose -lQtGui -lQtCore ; rm -f a.out
GNU ld version 2.18.50.0.9-8.fc10 20080822
  Supported emulations:
   elf_x86_64
   elf_i386
   i386linux
using internal linker script:
==================================================
/* Script for -z combreloc: combine and sort reloc sections */
OUTPUT_FORMAT("elf64-x86-64", "elf64-x86-64",
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OUTPUT_ARCH(i386:x86-64)
ENTRY(_start)
SEARCH_DIR("/usr/x86_64-redhat-linux/lib64"); SEARCH_DIR("/usr/local/lib64"); SEARCH_DIR("/lib64"); SEARCH_DIR("/usr/lib64"); SEARCH_DIR("/usr/x86_64-redhat-linux/lib"); SEARCH_DIR("/usr/lib64"); SEARCH_DIR("/usr/local/lib"); SEARCH_DIR("/lib"); SEARCH_DIR("/usr/lib");
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==================================================
attempt to open /usr/x86_64-redhat-linux/lib64/libQtGui.so failed
attempt to open /usr/x86_64-redhat-linux/lib64/libQtGui.a failed
attempt to open /usr/local/lib64/libQtGui.so failed
attempt to open /usr/local/lib64/libQtGui.a failed
attempt to open /lib64/libQtGui.so failed
attempt to open /lib64/libQtGui.a failed
attempt to open /usr/lib64/libQtGui.so succeeded
-lQtGui (/usr/lib64/libQtGui.so)
attempt to open /usr/x86_64-redhat-linux/lib64/libQtCore.so failed
attempt to open /usr/x86_64-redhat-linux/lib64/libQtCore.a failed
attempt to open /usr/local/lib64/libQtCore.so failed
attempt to open /usr/local/lib64/libQtCore.a failed
attempt to open /lib64/libQtCore.so failed
attempt to open /lib64/libQtCore.a failed
attempt to open /usr/lib64/libQtCore.so succeeded
-lQtCore (/usr/lib64/libQtCore.so)
libpng12.so.0 needed by /usr/lib64/libQtGui.so
found libpng12.so.0 at /usr/lib64/libpng12.so.0
libfreetype.so.6 needed by /usr/lib64/libQtGui.so
found libfreetype.so.6 at /usr/lib64/libfreetype.so.6
libgobject-2.0.so.0 needed by /usr/lib64/libQtGui.so
found libgobject-2.0.so.0 at /lib64/libgobject-2.0.so.0
libSM.so.6 needed by /usr/lib64/libQtGui.so
found libSM.so.6 at /usr/lib64/libSM.so.6
libICE.so.6 needed by /usr/lib64/libQtGui.so
found libICE.so.6 at /usr/lib64/libICE.so.6
libz.so.1 needed by /usr/lib64/libQtGui.so
found libz.so.1 at /lib64/libz.so.1
libgthread-2.0.so.0 needed by /usr/lib64/libQtGui.so
found libgthread-2.0.so.0 at /lib64/libgthread-2.0.so.0
librt.so.1 needed by /usr/lib64/libQtGui.so
found librt.so.1 at /lib64/librt.so.1
libglib-2.0.so.0 needed by /usr/lib64/libQtGui.so
found libglib-2.0.so.0 at /lib64/libglib-2.0.so.0
libpthread.so.0 needed by /usr/lib64/libQtGui.so
found libpthread.so.0 at /lib64/libpthread.so.0
libXi.so.6 needed by /usr/lib64/libQtGui.so
found libXi.so.6 at /usr/lib64/libXi.so.6
libXrender.so.1 needed by /usr/lib64/libQtGui.so
found libXrender.so.1 at /usr/lib64/libXrender.so.1
libXrandr.so.2 needed by /usr/lib64/libQtGui.so
found libXrandr.so.2 at /usr/lib64/libXrandr.so.2
libXfixes.so.3 needed by /usr/lib64/libQtGui.so
found libXfixes.so.3 at /usr/lib64/libXfixes.so.3
libXcursor.so.1 needed by /usr/lib64/libQtGui.so
found libXcursor.so.1 at /usr/lib64/libXcursor.so.1
libXinerama.so.1 needed by /usr/lib64/libQtGui.so
found libXinerama.so.1 at /usr/lib64/libXinerama.so.1
libfontconfig.so.1 needed by /usr/lib64/libQtGui.so
found libfontconfig.so.1 at /usr/lib64/libfontconfig.so.1
libXext.so.6 needed by /usr/lib64/libQtGui.so
found libXext.so.6 at /usr/lib64/libXext.so.6
libX11.so.6 needed by /usr/lib64/libQtGui.so
found libX11.so.6 at /usr/lib64/libX11.so.6
libdl.so.2 needed by /usr/lib64/libQtGui.so
found libdl.so.2 at /lib64/libdl.so.2
libstdc++.so.6 needed by /usr/lib64/libQtGui.so
found libstdc++.so.6 at /usr/lib64/libstdc++.so.6
libm.so.6 needed by /usr/lib64/libQtGui.so
found libm.so.6 at /lib64/libm.so.6
libgcc_s.so.1 needed by /usr/lib64/libQtGui.so
found libgcc_s.so.1 at /lib64/libgcc_s.so.1
libc.so.6 needed by /usr/lib64/libQtGui.so
found libc.so.6 at /lib64/libc.so.6
libuuid.so.1 needed by /usr/lib64/libSM.so.6
found libuuid.so.1 at /lib64/libuuid.so.1
ld-linux-x86-64.so.2 needed by /lib64/librt.so.1
found ld-linux-x86-64.so.2 at /lib64/ld-linux-x86-64.so.2
libexpat.so.1 needed by /usr/lib64/libfontconfig.so.1
found libexpat.so.1 at /lib64/libexpat.so.1
libXau.so.6 needed by /usr/lib64/libXext.so.6
found libXau.so.6 at /usr/lib64/libXau.so.6
libxcb-xlib.so.0 needed by /usr/lib64/libX11.so.6
found libxcb-xlib.so.0 at /usr/lib64/libxcb-xlib.so.0
libxcb.so.1 needed by /usr/lib64/libX11.so.6
found libxcb.so.1 at /usr/lib64/libxcb.so.1
libXdmcp.so.6 needed by /usr/lib64/libxcb.so.1
found libXdmcp.so.6 at /usr/lib64/libXdmcp.so.6
ld: warning: cannot find entry symbol _start; not setting start address
muelli@bigbox ~/svn/kvirc $ 









muelli@bigbox ~/svn/kvirc $ valgrind /opt/kvirc/bin/kvirc4 2>kvirc4-valgrind.log 
Segmentation fault
muelli@bigbox ~/svn/kvirc $ some excerpts from logfile:
==943== Syscall param writev(vector[...]) points to uninitialised byte(s)
==943==    at 0x3FB44DE4F3: writev (writev.c:46)
==943==    by 0x333A408BB0: _xcb_conn_wait (xcb_conn.c:170)
==943==    by 0x333A409178: _xcb_out_send (xcb_out.c:279)
==943==    by 0x333A409653: xcb_send_request (xcb_out.c:55)
==943==    by 0x4F6E116: _XPutXCBBuffer (xcb_lock.c:148)
==943==    by 0x4F6E43C: _XCBUnlockDisplay (xcb_lock.c:31)
==943==    by 0x333AC045E1: XRenderAddGlyphs (Glyph.c:111)
==943==    by 0x37F362F5A3: QFontEngineX11FT::uploadGlyphToServer(QFontEngineFT:
:QGlyphSet*, unsigned, QFontEngineFT::Glyph*, _XGlyphInfo*, int) const (qfonteng
ine_x11.cpp:1167)
==943==    by 0x37F363A20F: QFontEngineFT::loadGlyph(QFontEngineFT::QGlyphSet*, 
unsigned, QFontEngineFT::GlyphFormat, bool) const (qfontengine_ft.cpp:1141)
==943==    by 0x37F363B5A4: QFontEngineFT::recalcAdvances(QGlyphLayout*, QFlags<
QTextEngine::ShaperFlag>) const (qfontengine_ft_p.h:247)
==943==    by 0x37F36364D7: QFontEngineFT::stringToCMap(QChar const*, int, QGlyp
hLayout*, int*, QFlags<QTextEngine::ShaperFlag>) const (qfontengine_ft.cpp:1610)
==943==    by 0x37F3573083: QFontEngineMulti::stringToCMap(QChar const*, int, QG
lyphLayout*, int*, QFlags<QTextEngine::ShaperFlag>) const (qfontengine.cpp:1334)
==943==  Address 0x526179d is 7,765 bytes inside a block of size 8,680 alloc'd
==943==    at 0x4A05174: calloc (vg_replace_malloc.c:397)
==943==    by 0x333A408E2E: xcb_connect_to_fd (xcb_conn.c:224)
==943==    by 0x333A40B7F6: xcb_connect (xcb_util.c:332)
==943==    by 0x4F6E731: _XConnectXCB (xcb_disp.c:87)
==943==    by 0x4F5639C: XOpenDisplay (OpenDis.c:168)
==943==    by 0x6840DE: main (in /opt/kvirc/bin/kvirc4)
==943== 
==943== Invalid read of size 1
==943==    at 0xF357BDA: KviIrcView::haveUnreadedHighlightedMessages() (kvi_ircv
iew.h:150)
==943==    by 0xF352250: KviTrayIcon::grabActivityInfo() (libkvitrayicon.cpp:464
)
[...]
==943== ERROR SUMMARY: 9045 errors from 33 contexts (suppressed: 4 from 1)
==943== malloc/free: in use at exit: 24,427,666 bytes in 179,761 blocks.
==943== malloc/free: 2,553,601 allocs, 2,373,840 frees, 680,307,389 bytes alloca
ted.
==943== For counts of detected errors, rerun with: -v
==943== searching for pointers to 179,761 not-freed blocks.
==943== checked 23,399,520 bytes.
==943== 
==943== LEAK SUMMARY:
==943==    definitely lost: 197,826 bytes in 4,580 blocks.
==943==      possibly lost: 195,571 bytes in 695 blocks.
==943==    still reachable: 24,034,269 bytes in 174,486 blocks.
==943==         suppressed: 0 bytes in 0 blocks.

I'm attaching the full log.

I made it crash by clicking my own name in a channel widget...

Created attachment 360851 [details]
Valgrind log

Created attachment 360852 [details]
 valgrind -v --tool=memcheck --leak-check=full --num-callers=40 --log-file=valgrind.log /opt/kvirc/bin/kvirc4

muelli@bigbox ~/svn/kvirc $ valgrind -v --tool=memcheck --leak-check=full --num-callers=40 --log-file=valgrind.log /opt/kvirc/bin/kvirc4
X Error: BadMatch (invalid parameter attributes) 8
  Major opcode: 2 (X_ChangeWindowAttributes)
  Resource id:  0x86005a9
Segmentation fault
muelli@bigbox ~/svn/kvirc $ 

Again, it didn't crash itself (I probably didn't wait long enough), and I provoked it by clicking my nick in a channel widget.

So your Qt is consistently 4.5.2, no surprises there.

The Valgrind log shows a lot of invalid reads/writes in kvirc code and I'm fairly sure the corrupt QFont is also coming from those. I'm going to close this as not a Fedora/Qt bug, please report this (with the full Valgrind log) to kvirc upstream.

Thank you Kevin for your quick support!
I have opened a ticket here: https://svn.kvirc.de/kvirc/ticket/565