Thanks for the quick reply!
I did copy the whole thing, but then deleted it because it caused other problems. But maybe some polygons remained, I will definitely check again! Is there a way to highlight coplanar surfaces, like some programs can highlight duplicate lines, or similar? Or should I just search by hand?
Edit: I checked if there were any duplicate surfaces but didn't find any, though the render still shows the weird triangular lines. I also tried untriangulating the surfaces, after converting the current state to an object, but without any success
Edit 2: for anyone in the future who may see this: The problem never was with the polygon, instead one of the lights had the shadow map (soft) activated. After changing it to Raytraced (Hard) or Area, it works!