Thanks for all the comments, I was already aware of the login problem. The last software update to the Forum was installed in June. This addressed some security vulnerabilities but also contained some minor bugs which users are now experiencing. These bug fixes will be addressed with the next software update but at present it is only a beta version and not suitable for a "live" site. As soon as a "stable" version is available I will implement it (I cannot say when this will be). For now, please use the Login/Logout on the menu bar not the Guest Login at the top of the page.
TIP- Users don't need to log-in each time they visit the Forum. If you stay logged-in just close your browser when finished and the next time you visit the site you will still be logged in. When you log in remember to tick the "Always stay logged in" box.
Hope this helps.