I'm not using any hacks on my forum. Actually it is just a normal user title that is causing the problems (not rank like I said before - my mistake). Users with 0-100 posts have this user title. This is a standard vBulletin feature that's why I find it strange that it doesn't work.
It appears Classifieds cannot read correctly data that vBulletin has written into the database?
I checked my database and Classifieds actually enters these special characters in a different format as vBulletin. That's probably why there are problems...