Problem solved:
I've created a new line type from scratch (resources, select a line type, new line type, highlight "complex line type", highlight "page…", … ).
This works fine.
The custom line types I've used and that caused problems were made changing existing custom line types in my stationary files.
It seems to be that some of the custom line types need to be put in my trash.
How old are they? I think some of the resources were made using my first drawing program: MiniCad 6.
That's 22 years ago…