thanks a lot, your feedback did send me in the right direction to figure out the issue.
My issue was with the wire width of a specific net class.
which was corrupted (normal value is 6.0). So I’ve corrected my file in an editor. But I’ve no idea were this wire size width can be edited for a net.
I’ve just discovered, that you can now assign a net class to a wire in eeschma, and not only to a net in pcbnew.
You can set the wire width (for schematic) and the track width (for board layout) independendly. Schematic and board editor both have a setup-dialog. There is an extra pane dedicated to netclass-settings.