On single homed Airlock Gateways (as described in the network integration guide) the "set offline" button will not work on active/passive failover hosts if either "Private failover IP address" or "Mirror failover IP address" are in a network also used by any Virtual Host.
Use a dedicated, non-routed IP network for the failover IP addresses different from the IP network for the virtual hosts. The network interface itself may be shared.