This showed up when i was testing the joomla/phpbb3 bridge and it might not be an issue when using phpbb3 standalone. however, it's changed/broken from 3.0 to 3.0.1. When you install on a non-standard port (ie 8888 on my local install), during the install the hostname gets set wtih "hostname:port", eg "myserver:8888", where as in 3.0 it was just "hostname". There's a field for port below also. The problem i found was the cookie was no longer the same format as before, i thik the double port is getting assigned.