Received the patient and started investigations on it.
Apart from the inverted display there is also a, what looks like, random resetting of the processor. Some relays click, the screen goes black and the current drops. The screen going black is due to the brightness signal being turned off, but there are also moments that it stays on and the screen is just white.
Think this has to do with how far the processor can run through the code before running into trouble. The strange bit is that sometimes it starts up fine and works stable for a while, but eventually it turns of again.
The current is around ~350mA when the back light is off and no activity on the LCD signals, around ~750mA when the back light is on, but still no activity on the LCD signals. It goes up to ~1A when it is working.
The MCU clock shows normal signs of live, and the voltages are up to par without garbage on them. My guess is that the MCU is broken somehow. Ordered up a new one and when it arrives, I will see if it is indeed the culprit. It is a bit of a challenge with the ground pad, but some youtube videos show it can be done with just a hot air station.