security token
 

A user is complaining they can't post at one of my forums. They can post at other forums* but when they try posting at a particular forum* they get "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."

I logged in under their account and was able to post fine. Can you tell me what would cause this error? Would it be a problem on their end* since I could post fine from my PC?