This error has been plaguing us for months.
1) Nintex 2013
2) list with simple flexitasks and two approvals
3) workflow does not fail on start but after at least 2 days of waiting
4) all workflows do not fail but each day after 2 days waiting the likely hood increases. after 4,5,6 days the likely hood increases
5) workflow do not fail on weekends
6) workflows fail in larger amounts during the morning
7) workflow failure does not depend on ether approval task just time it remains un approved
8) this happens with all workflows on the entire farm
9) if i give permissions directly, using the person's name...almost no errors
10) if i give permissions with active directory security groups...largest amount of errors
11) using the permission checker...a large amount of users given permissions via active directory security groups report the user "DOES NOT HAVE PERMISSIONS" but they users can access the site and start workflow.