I think I had the same problem. I posted it on the old w3t site but I can't find it.
In my case I installed the perl version, registered - logged in, everything was fine.
I then installed the php version (different database), registered (same user name) tried logging in but no joy.
It fixed itself though. If memory serves, I logged into the perl version, opened another browser window (ie 5.5), and was able to log into the php version with no problems. Its been fine ever since.
Might be worth a try.
