Summary:
Have come across a TBOX User Input bug which I don't remember having been reported previously.

When playing a recording, if you press the YELLOW button (which activates a "close Caption" options popup window), DO NOT press the BACK button as this will lock up the TBOX user interface.

Details:
My series link recording of the 6pm news was active (recording began at 1750 and was scheduled to end at 1910).
Began playing the recording at around 1830.
33 minutes into to recording I accidentally pressed the YELLOW button, which brought up a popup with the following text:
+--------------+
|Close Caption|
|Audio English |
|Subtitles  off  |
|        OK        |
+--------------+
In a reflex action I pressed the BACK button instead of the OK button.
The screen went black, sound stopped, but the Close Caption popup remained on screen.
The dashboard lights remained power blue and recording red.
All remote control commands and dashboard commands were ignored.
I waited until 1912 to see of the dashboard recording light would go out, and when it didn't I rebooted the TBOX.
After the usual restart time, I selected the news recording for viewing and was able to resume from where the incident occurred (33 minutes into the recording).
The recording was complete (all 79 minutes), nothing was lost.

The fault is reproducible.
Hope this helps the techs fix the bug.