Checking in with the same issue here.
Only affecting the 3040’s that I’ve updated to 9.1 while 8.6 workstations continue working fine so I’m still thinking it’s somehow specific to ThinOS.
All Wyse terms are internal/local and do not pass through the Netscaler in our case. I can at least rule out the system time mismatch you proposed. I discovered that one myself during the last DST change so I know all our clocks are in sync now.
Thank you for the troubleshooting and ideas so far. Hoping we can get to a solution.