definately looks like you did not upgrade it as your copyright still says 4.1 from 2010
I would think you have not uploaded all the new files as the 5.0 release announcement has full notes. The notes above are what files to replace from 5.01 to 5.1 PhotoPost Classifieds 5.0 Released 05-29-2012
That would definately cause you lots of issues I would think if all the proper upgrading has not been done. If your logged in as admin you should see any true mysql error and post it.
Plus your style your trying to use some custom style you made from an old version. Clearly I think most of your issues come from not upgrading correctly. As noted in the announcement for 5.0 you must delete all old stylesheets and templates.