I gotten that when using "Back Arrow" for the last few months.
I think Dave put in some web security feature that causes it.
Sometimes I will "Back Arrow" twice, or hit "Try Again". Or maybe it was "Back Arrow" twice, followed by "Forward Arrow". I forget, but I think "Try Again" worked.
Invariably, if I follow a link away from where I'm reading, I use "CTRL + click" so the new link opens in another tab.
Like Vicki, I use "Return to the XYZ Forum" button when I;'m done reading a thread.