"Unable to load block into composer. You must edit this content from within the context of the page."

That is the error. Here's what happens. I have added this image page attribute to my this page type. Sometimes it works fine, but often the upload fails, and then a small popup with red background and some json content pop up (too bad I didn't copy the error, will update later probably). After dismissing of the error, everything is seemingly OK, I can re-attempt image upload. It seemingly uploads but then, when Composer is closed and the page is displayed - the attribute content (image) is not there. Returning to the composer, in the place of attribute, I see this error. I go back to the page, enter edit mode but this attribute block is nowhere to find at all.

I delete the page and start over. Very very inconvenient.