Based on information in this email:
   http://sourceforge.net/mailarchive/message.php?msg_id=29737312

On my local computer I tried using the version of roundup located at:
   http://hg.python.org/tracker/roundup/

with the issue template located at:

http://roundup.hg.sourceforge.net/hgweb/roundup/roundup/file/tip/website/issues

I created some dummy issues, and did some manual changes to the DB to 
"create" issues with large ids. All of the following msg snippets linked 
successfully:
   issue1
   issue12
   issue123
   issue1234
   issue12345
   issue123456
   issue1234567

So I am more included to think there is something weird happening on the 
servers instance... not really sure where to start looking though.