I have noticed this as well. We are running the latest available version of “Duo Authentication for Remote Desktop Gateway” on Server 2012 R2.
It happens randomly and a reboot fixes it.
As everyone else here, we are dealing with the same issue using RDGateway with DUO.
The fact that someone thought it would be a good idea to hard code an 8 hour limit without the ability to change it is ridiculous. Even worse, this isn’t a new issu...