I know this problem,not with the classifieds but with a other application.
Upload again all files from vbulletin 3.7.3 to your server.
It is possible that during the upload to your server one of the files will be corrupt throught timeouts or something else.
Is this not work go to the admin from bulletin and do maintaining on your forum. (Repair / Optimize Tables - Diagnostics).
I hope it works for you
Asking if you have any hacks/plugins would have been my next question.
We integrate with vb fine so hack issues are always something to consider. Thats why no company that writes software like vbulletin or us supports peoples code hacks. We can only make sure our software works as it is intended not what happens when one starts throwing in modifications.