Glyphs .notdef: Master is outside of the interpolation space at 100 and axis rage: 0–0

That means you either have special layers or Axis Location parameters that don’t line up. Double check those.

I don’t have any special layers. And I don’t get the Axis Location parameters —where can I change that?

Here is my Axis Mappings:

Don’t use Axis Mappings. Remove that. Set Axis Location parameters in all instances and masters.

Alright, Axis Mappings removed. Axis Location parameters set in all masters.
But how do I add Axis Location for a Variable Font export?

You don’t need to. Those are sourced from the instances and masters.

Okay, I changed some parameters and managed to export the VF in the end.
Thank you for the schnelle Rückmeldung! Happy Sunday!