URGENT !!!New members Unable to Log In

I am running 6.000.5

New users are now unable to log in despite being validating their email address and been activated via the Admin panel.

I created a test account and have the same problem. Older users can.

The log in page keeps appearing after each log in attempt , admin panel shows users as last visitied on log in attempt.

They appear as normal in any of the member's lists. They are just unable to log in.

Any help please........

Thanks sammie, I dont use chat on the site so that wasnt it. By some miracle I managed to upgrade to 6.1.2 and the problem has disappeared. I kept getting the "incorrect version" error message on upgrade attempts and I tried the upgrade again this am and it worked , no idea why .......................