TinyMCE loading error in Plone 6.0 after enabling/disabling custom theme

We are currently doing an migration to Plone 6.0 with a custom new theme package.
Problem: migrated content with RichText shows up properly in edit mode directly after the migration,
the same is true for the edit mode with our theme enable. However, TinyMCE show an error when we disabled TinyMCE again (using Barceleoneta again).

I think I see something fishy about that url. It has ++webresource++ and ++plone++static. Is that how it should be?

ps. I cannot ask of people to paste error messages when you start schlepping images of error messages too :wink: