OK, that should be fixed now.
One small typo in the code that checked for an existing login on the old pages (forum etc). The equivalent code on any newly rebuilt pages (home, mine search, mine details, album, photo and all the new registration pages etc) was fine.
It explains why having clicked a forum link (an old format page) and then being told to log in when you tried to post; that when going to the login page (new format page) it told you that you were already logged in.
Very confusing for anybody experiencing it. Sorry for any inconvenience.
my orders are to sit here and watch the world go by