Found pinout sheet plus maked some schematic drawing.
Maybe you could post the pinout and schematic for us to see and check.
What can be wrong? (wiring is ok, +5V & -23V are normal)
Sounds like it might be something about the sync signals. Or maybe some needed signal not connected or making proper contact. It's a bit difficult to say much without more information but my first step would be to look into any documentation and information available about the display and then how the display has been connected to the USB13700 controller board.
The one thing that looked suspicious was that the data lines in the pinout doc were in opposite direction according to my drawing (directly from IC)
If you have the D0-D3 in wrong order that should only make the pixels appear in wrong order but still get the picture.
P.S. Henri, how about a possibility of enabling I2C bus in the board software?
It has been been my intention to work on adding I2C support for USB13700. I think I could probably take some time to work on this sometime in June hopefully.