That doesn't sound like a classifieds error.
I am using a clean 5.21 right here have from the get go. Main Index - Reeftalk Classifieds
All our products integrate with vb4 the same way so I would need alot more info here. Not sure what our product would do to cause that error. If plugins are disabled definately do not see how our product could cause that since vbulletin has no idea classifieds exists. The plugins only show classifieds stuff on your forum.
I just do not see how our upgrade could do that. If you uninstall your classifieds plugins does the error persist?
I just reinstalled the plugins for classifieds in 5.21 and my site is fine.