It was fixed (once again) by unhooking the battery. This time at the jump-point under the hood. (As the boot wouldn't open.)
I have opened up the DIM before, resoldering all suspect solderjoints, only to find it was indeed just a fuse... (not linked to the first or second failure of the DIM in conjunction with the indicators not working and the central locking not functioning, but a third occasion when the DIM just didn't light up but I had no other errors)
Hence my fuel gauge having an offset
You have to ripp off all needles of the dials beforte you can open it and putting it back in the exact same position is hard.
From everything I read, it is, as Outnumbered answered the first time, a CANBUS issue.
If it does happen again, I will measure some voltages on the datalines to see if it is some grounding issue, but I doubt it as it is solved by simply taking the voltage of off the cars electronics.
There is quite some sparking when I reattached the battery. So I guess all computers draw a significant amount when first seeing voltage after being cut off.
Once again, I think it is VERY POOR designing of Volvo that ANYTHING can cause the mechanical opening of the doors to fail. I simply cannot understand the design-choice made here.