When I expand forums to see sub-forums I get it loads the entire page inside the element like in the picture below. 

I believe the ajax response is not filtered properly and is having some issues. Also the menu on the top right of the page stops working after this

PS: I recently purchased this but I got an email telling me to post the issue here.

PS: I think I did a reset from the first one

Please check for mod_security or securi, and check its not blocking something there. Cloudflare is another thing that may cause this