Amanda uses UDP packets to yak between backup servers and clients.  The
default maximum packet size is pretty small (see MAX_DGRAM in
common-src/dgram.h).  It is so small in fact that a client with more than a
couple of dozen backup directories can fail to participate in an amanda
run.  This problem is known to the amanda developers, and as a quick
workaround, they have suggested upping this limit to near the TCP/IP
maximum.  So, consider applying this
pseudo-patch:

common-src/dgram.h:
-#include MAX_DGRAM    8192
+#include MAX_DGRAM    65500