I don't have an easy fix. If it where me I'd try to back up my forum files. Find out where the posts are kept, maybe sql table. Then reinstall everything. But keep back up files in case there are any problems. That's the hard way but I bet a sure fix.
Or try to just disable then delete the plugin and reinstall it. But make sure to do back ups. I don't see the plugin at the store since it is a default. You will have to redownload oxwall to get that plugin.
This is the only way I know how to fix it. I'd wait for others to comment.
Hope I didn't confuse you.