The UPL was designed to run without the optional keyboard. I do keep one plugged in most of the time because it makes building new setups possible with file naming and so on.
The connectors are absolutely compatible, otherwise my modern keyboard would never have worked with the UPL.
I have it booted up now, and I have decided to just leave it powered on 24/7, rather than shut down and lose all the setup again. It takes quite a while to disable a bunch of controllers and ports (the motherboard controllers conflict with the digital board's controllers, preventing the HDD from being detected).
Why they chose not to use the normal HDD controller for the disc storage, opting to put it on the controller on the digital board, is a mystery. Had it been on the normal controller, this type of fault would not keep it from booting.
The on board stuff has to be disabled. All of it. The drive specs have to be entered each time.
Oddly, only the date and time are being retained now. Everything else is gone when the power is off.