OK
I did all those things and things worked as expected for the last 4 days. Then tonight it failed again when I hit the back button and clicked on the "Mark Forums Read" at the bottom. Got the same message "Your submission could not be processed because a security token was invalid. If this occurred unexpectedly, please inform the administrator and describe the action you performed before you received this error."
O then went back into the forums and clicked on the top right pf the black menu bar and then it worked.
Since it was always only occasional, I'm not sure it was really fixed before the new failure.
Rg