Thank you, Brad -
I’ve finally managed to try a different lightening adaptor with the PX-S3000. To my great disappointment, the second adaptor didn’t work either.
Also: deleted app, re-downloaded Chordana Play for Piano, put the iPhone into flight mode, fired up - and same error. Also tested with iPad, and a friend's iPhone (he is a professional DJ and very experienced with MIDI etc.), same error.
Nobody has mentioned any possible setting issues, and I’ve not found anything in the system settings that makes the keyboard and iOS devices work via USB. Bluetooth playback works flawlessly, I should add, but this is irrelevant to the problem.
Thank you for any further advice. I’m afraid that here in Israel it is very difficult to get expert service.