Did you "reinstall" the forum script in the new location, or did you just move the files?
If you can't 'undo' what you've done, and if you're not conversant with mod_rewrite coding, I'd suggest you hire someone familiar with your forum script to help you move it. Otherwise, consider putting it back where it was. We can handle specific questions about .htaccess file and mod_rewrite coding here, but we're really not set up to be a 'help desk' for your forum script... :(
try backing up the database and having a look at it in a text editor. see if there are any URLs hardcoded into it, pointing to directories outside of the forum directory, or something like that. maybe these URLs are now pointing to the wrong place.
I'd advise looking at the code in the .htaccess files, and finding out why a trailing slash appears to be required. If you change your links, you thereby change your 'real' URLs. If you change your URLs, then you may expect those new URLs' rankings to suffer... and you'd likely want to add code to redirect the old URLs to the new in order to avoid losing traffic.
Fix the problem at its source instead of trying to find a 'quick fix.' Quick fixes tend to have unexpected and unpleasant side effects, especially as regards search results.
Nothing but a top-down analysis of 'how your site works' will do in this case. Start with a link on a page, and work through the .htaccess processing that handles the linked URL when it is requested by the client from your server, through its translation to a server filepath, and any handling of that URL within whatever script that URL gets 'delivered to' as a result of the .htaccess processing.