Hi Christian,
Thank you for your reply.
Indeed the simplification of the polygons helped a lot. The import-export method you described sounds very interesting to me, I'll be trying it next time if I still get errors.
I discovered that my outer domain boundary followed the raster pixels (raster to feature convertion) which lead to the creation of numerous tiny triangles. This made the triangulation fail or at least it took a lot of memory. Then I also included a minimum distance (buffer and clip) between the outer domain boundary and the building polygons to make sure that there are no overlaps. With this I could generate the desired mesh and this question can be closed.
If I may ask, do you have an idea for my question about the break lines in the mesh generator (see general topics, "import of break lines")?
Best regards, Sam