Sorry for the late-ish reply. I haven't been visiting GTP for quite some time, so I can not reconstruct how long this has been going on.
I get the "security token invalid" message when I use my bookmarked "mark forums as read" link. When I manually click the link at the bottom of the forums overview page, it works fine. When I bookmark that very link however, it doesn't work.
I am on Firefox 3.5.9 and WinXP SP3. Will log out, clear the cache, log in and see if the problem persists now...
EDIT: yes, I'm afraid this problem persists. Funnily, I have read of the exact same problem from member of a completely different (yet also vBulletin-based) forum only days ago. Maybe one of the latest forum updates has a bug?
EDIT #2: I just bookmarked a fresh link, accessed it from my bookmarks, and it worked. I will monitor this problem and report further issues.
EDIT #3: So the newly bookmarked "mark forums read" worked for a while, but now runs into the "security token invalid" error again. Is it possible that the forum has to have the "markreadhash" value in order to mark the forums read, but said hash value changes in planned intervals? That would mean that one can not bookmark this link, because it will be invalid after a while.