@sven , following your advice, I removed the SIM card and restarted the vollaphone. The problems did not go away. I restarted again and tested again in a time of about 24 hours. The result was that the problems did not go away.
Now I use my vollaphone with the SIM card again. But I really must find a solution, because as time goes on the phone gets more and more unusable.

@Ian , maybe it's a problem of the original Vollaphone and not of Vollaphone X. Or something, which we did not find out yet, triggers this problem after some months of usage. I also did not have these problems from the start.

Has anyone found a solution (or a workaround) for the ghost touches problem? (The funny thing is: I already owned my Volla for a year before the problem surfaced. Now, with time or with each update, the problem seems to get worse)

@Ian , thanks! I found out that Biometrics are disabled on my phone and that there is no finderprint ID installed. So the finderprint scanner cannot be the reason for the ghost touches.

I have problems with these "ghost touches", too. I tried to find a way to disable the fingerprint sensor, but I cannot find anything about this on my Vollaphone or in the internet. Where can I disable the fingerprint sensor?
Has someone found a solution for the ghost touches problem yet?