So it sounds like the user gets their IP->User mapping successfully for group access when they log in, then they lose the mapping early. There are several causes for this:
1) Watch out for automated service accounts firing up on the user's workstation, logging into the domain, and overwriting the user's mapping. You should be able to see this in the logs for subsequent connections from that user's IP, and the user name will show up as "backups" or whatever the service account name is. Make sure all service accounts like this are excluded from forming mappings on the gateway/cluster object under AD Query...Advanced...Excluded Users/Machines.
2) Check the gateway/cluster's User/IP Association timer and make sure it is greater than the kerberos reticketing interval for AD. Usually the AD interval is 8 hours (with the firewall set for 10 or 12 hours) but I've seen cases when the AD administrator increases the reticketing timer to reduce load on the controller, and users lose their mappings early.
Note that if the user locks their screen and unlocks it with their domain credentials, their mapping should be reestablished and it is probably one of the issues above. If that doesn't get their mapping back there are deeper issues at work here.
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