So, continuing with #DecentSampler on #Apple. Well, the Decent Sampler runs smoothely. But this iPad MIDI thing I don't get to run.
Another option would be to use my old #MacBookPro 2017 with #OSX. Decent Sampler works out of the box on the old OS X 12.x.
I plug in my MIDI-USB source, which is a Roland GI20 guitar-to-midi system. OS X ignores it. I plug in some USB-MIDI-5pin adapter. OS X ignores it. I plug in another of these adapters/midi interfaces. Nothing.
Turns out in the end that the Roland GI20 does not work with Apple products at all. It only works out of the box with #Linux. For all other OSses it needs special drivers.
Apple offers me an update of IOS 15.something, and with one of the USB-MIDI-interfaces plugged into the oldskool MIDI port of the GI20 I can now finally use DecentSampler on an Apple machine, funnily on IOS 15, which fails with many MIDI applications such as MixingStation + fader controllers.
However, this iPad is too weak for DecentSampler.