Thank you Yann for your response.
The issue is in the Editor in general (Design, Templates, Library, etc.: Appearance > Editor > Design
I have Gutenberg and Twentig activated on multiple sites. Only on the multi-domain site with subdomains en. fr. de. does it give me this error.
Gutenberg and Twentig are up to date.
I actually really like Twentig because it is light and does not overwhelm the site like other block builders that feel like a bad case of herpies ;-).