I got this error while trying to enter Login :
The requested URL /phpBB3/login.php was not found on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
It occurred because I tried to bridge cpg 1.12 to phpBB3. Before I tried to finish building the bridge I got a warning message. I read the warning instruction about going to the bridge ? file and doing ? in case things failed, but clearly failed to assimilate the warning. I've tried to find that instruction about how to rescue the situation, but wasn't successful. Now I can't get in repeat the mistake!
The phpbb3 bridge is a user contribution and goes unsupported. Read up the corresponding thread (http://forum.coppermine-gallery.net/index.php/topic,43383.0.html).