I guess this is a non-Theme issue but I hope to find a solution to the following problem:
Somehow while migrating my site the widget block editor disappeared. The whole window where the individual widget areas should appear is empty.
Editing the widgets in the customizer is still functioning but in the backend block editor it is somehow broken.
Deactivating all plugins, reinstalling WordPress and switching themes brought no result.
I used Better Search & Replace to change the domains in the database (without having saved a backup of course 🙁 ).