transparent grey loading screen for all of eternity

Every time I want to add a gallery, images to a gallery or if I want to open the Options / General Settings in the left side menu a transparent and permanent grey loading screen appears as overlay and blocks any interactions. I have to close the page or click “back” to get out.
I disabled script blockers and all things that could have interfered… I also followed your HELP instructions about the REST API: it works fine. I am at my wit’s end, what can I do? I use WP 5.5, until a month ago or so the plugin worked fine.

I also encounter a problem with inserting media from the media library in pages, nothing happens if I click on “Add Media”.
Maybe the two issues are related..?

The page I need help with: [log in to see the link]