Symptoms

This morning when users tried to open their tasks they were greeted by the error "User does not have permission to open this task"
 

Diagnoses

During our investigative meeting, we found that the Active Directory admins changed something in the Active Directory for the two domains R1 and R4 (which contain all users that couldn't access their worklist tasks).
 

Resolution

K2 didn't have time yet to update all users in the Identity cache, so we ran the Force Identity Refresh Tool (available on the K2 Marketplace) and forced an immediate refresh of the "R4Domain Users" and "R1Domain Users", after this was done the users could login again and start processes.

I suggest that you speak with your AD guys to please give the K2 server at least 8 hours time (depending on the Identity CacheConfiguration Settings) to run through all the users in the Identity cache, in other words rather update the AD over the weekend or so.