I had VBulletin and I am using Xenforo years ago. I have htaccess redirects and everything was working perfect until now, I went from 1.3.x to 1.4.2 and now I have thousands of 404 errors from all the old stuff.
This URL appears to be a phpBB URL. The only way that would be redirected is if the viewtopic.php file exists and is handling the redirection or if you did something in .htaccess to do the redirection. I can't tell you what you did, but presumably one of those steps was undone.