vB 4.2 login problems

[Blocker]

Hi,
Having problem when logging into my forum. Although I have logged in as administrator or standard registered user, the forum does not show I am logged in.
It is only when I click into a thread that the forum recognizes and displays that I have logged in.

Example;

• Land on forum home page - empty log in credentials and banner appears "If this is your first visit be sure to..."
• Click on any thread and I have access and username appears top right link.
• Close or leave forum and return - I do not appear to be logged in again.
• Even after logging in, the 'If this is your first time.." dialog appears, but clicking on a thread logs my in.

Has anyone seen this? Any solution?

Thanks