Hi, I have wp-united, too, and it works with Advanced SEO URLs.
But I have nginx, so I can't help you with htaccess, I only can suggest you should triple check htaccess rules, every string. In my case I had to write 3 my own rewrite rules (regexp strings) to get it working.
If you place your htaccess into your /forum/ dir, you mustn't have '^forum' in your htaccess file and you need to delete previouse htaccess rewrite rules at the root of your website.
Also try too look into your logs, especially to error-log of your website, try to change log level to notice or something like that (sorry, I have no apache, but philosophy is the same).
I beleive you'll get this mod working.. and then, please, let me know if you'll get an issue like mine: Problem with virtual folders and avatars