from the display driver, the serial communication seems quite stable. We can now download files using our GSM modem over serial with no problem. We of course also get the same good results if we detach the screen completely (as you suggested).
However, sometimes when we close the serial connection, reboot the GSM-modem and open the serial connection again, the random-bytes-disappearing-error can appear. But if we again close the serial connection, reboot the modem, and open the serial connection when that happens, it seems to recover and we can (again) communicate with the modem without missing bytes.
So generally this is very good news for us since we are not using the touch interface on our production hardware. So by not initializing touch it seems that we can use G400 instead of ChipworkX now.
@ Dat - Yes, I too think it’s another problem and not that critical for us, since we can work around the issue by resetting the GSM module (again), when it occurs. But thanks, I’ll try your suggestions and see if it helps.