Problem with user database

Hi,

I recently installed punBB on my server and i wanted to integrate it with my existing user-system. Since my users table and punBBs users table where quite the same structure i could just copy & paste. If the users login to my website, they will also be logged in to the forums with an hidden form. I got a big problem: If the users log in with my system, they are logged into the forums without any problems, but once they log out and want to log in again. It wont work. My own login system says that the passwords do not match and punBB says that a cookie could not be created. I looked into my users table and the passwords of the users where changed! I use a PHP session to store all the information and MD5 hash. Am i doing something wrong?

Thanks in advance!