I find that if I go direct to www.blackoutaudio.co.uk/forum, I get that problem - I can enter my details into the login box, and it seems to log in, but then when the page loads, I still see the login box at the top.

However, if I go to www.blackoutaudio.co.uk homepage, and click on Forum at the top, I can log in and stay logged in.

Dunno why that is, but is a workaround for now!