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