More data: I've narrowed down the problem for me to the "Go" button at the bottom of a thread. It seems that all other navigation works OK without logging me out.
Clicking on one of the breadcrumbs to go back to the sub-forum top or a parent sub-forum works OK
Clicking on the "Jump to..." drop down list at the bottom of a thread works OK
Clicking on the "Go" button at the bottom of a thread DOES NOT WORK AND CAUSES A LOGOUT
This is probably why some people are working OK...they probably do not use the "Go" button for navigation. For now, you can stay logged on if you don't use the GO button.
Dave
EDIT: Well, it looks like posting a message also causes a LOGOUT.
I was logged out immediately after posting the above.