New registered members can not use the forum. every time they try to interact (either post, reply, or check the Private Message) they get a message saying "you're not registered blah blah".. But they are registered, although their username doesn't show up in the main page "Ultimate.cgi" nor the list in the CP, but I can see their files inside the Members directory.

This is my UBB,

http://www.demna.net/cgi-bin/Ultimate.cgi

One last question, what does the "Rebuild Member Files" function do in the Misc. and Debugging link inside the CP? (will it help fix my problem?)

It does not hurt to rebuilt member files (trying) although I can't guaranteee it will work.

Did you re-upload the member files? Because when i did that, I found there username in the CP, but they couldn't post coz it said they werent registered.

Arrgh!..

It was a permission problem. The Memberlist file inside the Memebers directory was set to 666 I think it should have been 777.. now it works perfectly.

Thanks guys

Better remember that for next time when it happens to me again and I go crazy.