OWA kicks user out within seconds of authenticating

Issue exists for all users at a specific location.
Users accessing same system at other locations do NOT get the same issue.
When kicked out, there is no error, OWA dumps the user back at the login screen.
Issue has been replicated on Chrome, IE, Edge, and Firefox.
Cookie handling is WIDE OPEN.
HTTP and HTTPS are unrestricted by the Firewall in the Router.

Any advice would be Very much appreciated!
I'm desperate :D

• Ian Hawkins