When I add a link to another ticket inside a comment block and submit the changes the server will return an internal server error with code 500.
When I reload the page it is correctly displayed and the trackback links are created correctly.
Here's the section from the server logs:
access.log:
10.53.4.180 - - [12/Aug/2008:10:28:31 +0200] "POST /trac/_sandbox/ticket/9 HTTP/1.1" 500 613 "http://myserver/trac/_sandbox/ticket/9" "Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.1) Gecko/2008071618 Iceweasel/3.0.1 (Debian-3.0.1-1)"
error.log:
[Tue Aug 12 10:28:31 2008] [error] [client xxx.xxx.xxx.xxx] malformed header from script. Bad header={{{: trac.cgi, referer: http://myserver/trac/_sandbox/ticket/9