phpBB Sverige

And lastly, we have noticed users who for whatever reason did not have the updated files. In some cases, this was due to a webhost error, other times an FTP client messing up, or something else happening. Regardless of the cause, users need to ensure they have the latest files. There are several ways to do this, such as checking the timestamps on the files, checksums, etc. However you do it, this should become a habit everytime you update.
This will not only ensure you are protected from the non-security related bug, or a vulnerability, but help those supporting you to get you the answer that much faster.

If you are having problems logging into the admin panel, this is probably the issue. You should make sure that all files are up to date, but login.php, includes/session.php, includes/functions.php and admin/pagestart.php are the most relevant.