I have had Drupal 6 installed for some time now and it was working ok, recently I enabled the contact form module and I have noticed a problem so disabled it but unfortunatly the problem didn't go away.

The problem is:
When an authenticated user logs in they can carry out normal functions ok but when it comes to logging out a problem arises, Drupal wont let them log out and they remain stuck, what happens is when a authenticated user clicks log out they are redirected to the logged out page where their username dissapears and turns into a log in button. However if they click log in again they are directed to a page showing them as already logged in from previous log in with one difference, they cannot do anything without receiving error message - "Access denied"
So it seems that authenticated users are not being logged out correctly but all others such as admin are.
The only way around this is to clear the cookies and browsing cache which isn't very convieniant, I think it may have to do with caching and the cookies but don't know how to fix it.

Does anyone have any experience of this?
Thanks in advance for any help offered.