Fatal error: Class name must be a valid object or a string in /home/dsmedia/blog/public_html/includes/common.inc on line 7749
I had a similar issue today (same error message) and I just solved it. It concerns my personal case, but if the description can help anyone : 
the problem was the fact that I had created a few items with a content type that I deleted since. So these non published items had an invalid class, because the content type was not existing anymore. 
I found these 3 items in the list of content with a blank space instead of the content type name. 
Deleting these 3 items, and then extracting/importing the database again to the customer made it work. 

The stange thing is that my version of the site was still working since I deleted the content type, so I couldn't understand why suddenly this problem when migrating the site.