Configuration required. Please fill in data in all elements

When editing a shopping experience landingpage and try to save, I get the error notification: „Configuration required. Please fill in data in all elements.“ The error message is misleading and not helpful, as it suggests that some from entry might be missing, while the actual cause is a failure to upload the image (see details below).

Unfortunately, it does not mark anything as incomplete, so I can only guess. I tried to fill out everything, even optional stuff like CSS class, on any elements in any viewport, still this message keeps coming up.

In my dev tools, there is an error message in the console:

This seems to be a subsequent fault, when something goes wrong saving the image. The saving fault does not trigger any error notification, but instead the success notification „image saved“ is missing.

Image saving fault:
500 POST /api/v2/_action/media/dd6d3e25f84d4278b985142a3cb3cded/upload?extension=jpg&fileName=testimage

When retrying with the same image several times, somehow the error went away and I was able to save the theme customization again.