I’ve done a little more digging and turned on the more detailed logging to find the following entries:
9:39:54 AM Gateway No session found for authenticated gateway call: 80
9:39:54 AM Gateway No session found for authenticated gateway call: Alert
9:39:54 AM Gateway No session found for authenticated gateway call: SQLTags
9:39:53 AM Gateway No session found for authenticated gateway call: RunQuery
Perhaps a server reboot would fix this problem?