Now I improved the issues in my lef file (which gives much less warnings), but now it crashes a bit earlier in RePlAce: http://pdk.libresilicon.com/caravel-build3.html

Since you're design is considered a small design, you might want to check this: https://github.com/efabless/openlane/wiki#how-to-add-a-small-design

I tried all those configuration changes, but OpenRoad still crashes with a segfault.