security token was invalid
 

I am running vBulletin 3.8.5. When a user clicks on "Quick Links" and then on "Mark Forums Read", they get the following error.
Code:
Your submission could not be processed because a security token was invalid.
I have not installed anything new lately, so I do not know where to look to find the problem.

I would revert a template if I only knew which one to revert.

Ideas?