Recent vim versions can no longer process etags-generated tag files
that worked just fine with older builds.

Reproducing the issue is quite simple:

  1. Create a trivial C program.

     $ cat >test.c <<EOF
     void foo() {}
     void bar() {}
     void baz() {}

     inf main(int argc, char **argv)
     {
         foo();
         bar();
         baz();

         return 0;
     }
     EOF

  2. Build tags database.

     $ etags -R

  3. Try jumping straight to a tag.

     $ vim -t foo

     The error message will be:

       E434: Can't find tag pattern

  4. Alternatively, open the C program and try jumping to a tag
     using Ctrl-].

     The error message will be one of:

       E48: Not allowed in sandbox: U
       E492: Not an editor command: ^?
       E435: Couldn't find tag, just guessing!

     with no apparent correlation between the specific error message
     and the action performed, that is, repeating the same steps
     three times in a row might result in all three error messages
     being displayed one after the other.

I've "bisected" the issue and found that

  vim-8.0.1187-1

works fine, whereas

  vim-8.0.1322-1

presents the issue. I've tested this both by gradually upgrading
two Fedora guests (25 and 26; freshly installed, so that no vim
customizations would be involved) and by downgrading my laptop,
which is running Fedora 27 and exhibits the issue, to vim-8.0.1187-1
upon which the functionality started working once again.

etags is ctags-etags-5.8-21.fc27.x86_64 on my laptop.