Thanks - the screen was fine. I had 20 of those, so I could test a few 😉
But an update:
A fellow member sent me the instructions and files he received from F(X)Tec - and it worked fine!
And the best thing is:
It's really painless if you have a rooted OS. No debug firmware needed, no reflash, no wiping of data.
Just one single APK, a firmware file and access via adb to change some permissions for the touchscreen driver files.
So if anyone has the same issues: With a rooted OS, it takes 2 minutes to fix it 😄