Not sure what to tell you Disco...it's working on my dev dedicated, and a test listen server ...perhaps another mod is somehow conflicting.
Even with that being said, a map change I think would still create the file and say 'map changed' every time that happens, even if no other things get logged.
Other than permission issues, I can think of no other reason logs wouldn't get created than some type of conflict.
Might try deleting addons/ULX and addons/ULib, /data/ulx_logs, then Re-svn get ULib and ULX and place them in.
You are hosting a dedicated server, correct? And you're looking on the dedicated server data/ulx_logs directory...not local client?
(I expect you aren't, but, hey, have to ask...we're only human)