I had the same problem plus I had a problem with my search function. When I followed Drexion's advice below, it solved the logging-in problem as well as the search problem. I hope that someone makes this a 'sticky' thread so that people will be able to find the answer quickly and easily. That one 'functions.php' file was the only one that had to be uploaded.
Did you edit any of the php files ?
Re-download phpBB2.0.11, and extract it to your hard disk. Inside the includes folder there should be a 'functions.php' file - upload this fresh functions.php into your forum's /includes/ folder, overwriting the old one there.
Thanks, Drexion! I found your reply to this problem on the following page:
http://www.phpbb.com/phpBB/viewtopic.ph ... anusername
It solved my problem immediately, and as I stated on that thread, I think that you should make this a 'sticky' thread so that others can find the answer quickly.
I was really hoping that it would be a quick and simple fix, and thanks to you, it certainly was.
It also solved my 'search' problem because I was getting a similar type of error when I tried to use the search function on my forum.
Thanks again, Drexion!