Think this is the same problem pantontino was experiencing.
The problem is because we allow anonymous (guest) posts, - when you reply to a thread and you're not logged in (ie, the system thinks you're not a registered user), you are given the option to enter a name, rather than just being shown as "guest". If that name clashes with a registered user, then you have to use something else in order to prevent people posting anonymously whilst masquerading as a registered user.
So, two options. Either we don't allow anon posts, which I'm not sure is the best way to go, or registered users must log in first.
An easy solution, if you have cookies enabled, is just to tick the "remember me" box, and then you should get logged in anyway each time you return to the forum.
Hope that helps??
