So I'm back with an update; I figured out something weird. When the remote keys do not work and I put the key in the door lock, the lock turns just fine. While the key is in the door lock turned to the left (to open the lock) and at the same time I press the lock or unlock button on the remote key, it suddenly works again! I don't have to program the key into the car anymore, it just works.

There's a thousand idea's and thoughts on what this could explain, but nothing that really fully makes sense to me. I'm hoping one of you guys with more experience can tell me what this might mean.

__________________
2002 BMW E53 4.6is - Estoril blue
1984 BMW E30 325 - Zinnoberrot
1964 Chevrolet Chevelle 300 - Meadow green