I went through the blocks and checked as many as I could, but at 2300 they are just too many.
It seems that if I place 1 block and copy it 10 times over next to each other, some of those blocks produce "duplicate plane"-warnings and some don't (which makes me even more confused about this).

The only "solution" that I found is, taking the detail out of the map, deleting blocks, reducing everything. That is obviously one of the worst solutions but I can't trace anything back.

I will keep an eye on this, I hope I can still figure it out and then get all my map objects back