Looks like you're not patched to 1.3.  I had UT3Fixer on the server which will help people with 1.2 or below but it fucks up the teams so I took it off.  So you have to be on 1.3 in order to join mid-game when a custom map is playing.

Download the patch here:

http://download.dfbgaming.com:9080/UT3/UT3Patch3.exe

If you have 1.3 already or the patch doesn't fix the problem let me know.  This should fix the problem though.