Default [Q] WiFi Tethering: ERROR

I recently switched to TMobile from AT&T. I have verified from them that I should have working Tethering; that is, they claim there is no block and that I have 5GB of data available per month on my plan.

However, I am not able to turn on tethering.

When I go into the tether control and click it, my WiFi turns off, then I briefly see the "turning on WiFi Hotspot..." Then the message becomes "Error."

That's it. Wifi won't toggle back on, so I need to reboot to get WiFi back.

Any ideas?