I checked the manual for a description what the error code for the blinking lights meant. The error obviously wasn't mentioned.

I then went to google that shit and found out that the pump might be blocked.
I took the dishwasher apart and found indeed stuff blocking the water inside and pulled it out.
I reassembled the whole thing and put it back in my kitchen and gave it another try.
Another error appeared. This time it's supposed to be a leak in the pump. So I disassembled it again and tadaa it seemed like I broke some plastic hook of the pump when I first got my hands on it.
Ordered a new pump for 30€.
Next day I break that thing apart one more time to get the new pump in. Doing that I notice a filter on the floor that I must have missed the first two times.

As I really didn't care how the thing was looking before I started messing with it, I just figured that it had to go SOMEWHERE in there. So I put it into the pump as well.

Assembled it again - I got some routine now of the whole process of disassembling and reassembling with dragging the machine out of the kitchen, so that didn't take longer than 15mins each from now on - ran it on the short programm and it worked! Hooray.

So I started the normal programm and had it fail again. Taking it apart once more, and found the filter that I put on the pump to be jammed with stuff and once more blocking the water flow. So I removed that again, reassembled and turned it on.

Didn't work at all. I got it out once more because I remembered that I probably not had connected the electricity for the pump again - and so it was.

Got it back to gather thinking that all of this must end now soon...
Short programm ran without errors.
Normal programm stopped with the regular error. NARF.
Then I had THE idea and ran another longtime programm instead of the normal one and tadaaa it worked.

So it seemed to be a broken circuit in the normal routine for the machine. Avoiding it now. Saved me 400€ for a new one, but got me some grey hairs.

After all of that my INXJ wife told me that, she'd remembered where each part was located in the first place and would have probably left the machine outside the kitchen itself to run the programs for testing.

I said:
But where is the fun of that?