Interesting. I have converted keyboards both ways many times (and from AT to PS/2 to USB) and never had any issues.

I've never come across a single din keyboard not working with a simple din->ps/2 adaptor. I assume you've already tried connecting a normal ps/2 keyboard to that port, just to make sure nothing else is wrong? If you have, then you have a keyboard with a very unique personality there 😀 (which might not be so strange after all, it IS a weird keyboard with the trackball and all)

I think the converter is damaged.
I will try to find another one (there used to be tons of these dirt cheap).

Yeah, it also lacks arrow keys and function buttons 😁

Is the D-Sub for a serial connection? Is that just for the keyboard, or is it for the mouse component? (are they separate but share the same plastic body, or are they meant to share the same connector to the PC?)

Where I'm going with this is, does the keyboard/trackball combo work if connected via serial or DIN (or what have you)?

Serial connection is just for the mouse. The Keyboard works with out it connected, but i think the mouse draws power from DIN connector for the Keyboard.