LAST UPDATE.
I close the thread because the issue now definitely seems resolved. Keys are working as they should. Killing the X server MIGHT have been the solution.
Still, I did experience lapses where keys reverted to non-responsiveness.
I also did another adjustment yesterday, though, after which and ever since the keys have worked flawlessly. Some months ago my daughter “watered” my laptop. I bought a new one and had the shop transfer my entire system to the new laptop. I noticed that the old host name was also present in my new laptop.
So I did the procedure described here:
After that everything worked as it should.
Thank you for your kind attention. Regards, R.