Basic URLs with parameters (?p=123) are working, neither do the nice.
I can't get this sentence to work. Is there a missing "not", a superfluous n, or did your cat walk across the keyboard?
When you say "not working any more"*
what exactly do you mean? Yes, I know it's transparently obvious to you because you've been fighting with it for days, but it's new to us. What did it use to do that it doesn't do now?
As written, the RewriteRule would take
any incoming request for
any file or folder that doesn't exist, and route it to /folder/index.php. Are you absolutely positive that's what you want? In particular, are you certain you want a rewrite rather than a redirect? With a rewrite as coded, every single user will think they've entered the correct address, because they've all ended up in the Forums. With a redirect, they will see the correct address in their browser.
See thread in the immediate vicinity of this one for complete boilerplate on what happens to query strings. (Short version: Nothing, so you don't have to think about them.)
* You know you're getting old when you cringe at "anymore" written as one word.