I read that it was possible to upload at the start of the hour, and for a period of time after that. I can confirm that that workaround worked for me, and apparently others. I was trying to upload a couple minutes before the hour, and each time It was failing. As soon as the start of the hour arrived the upload went through correctly.

The workaround .compositingGroup()that I used, no longer works, at least it is not working with iOS15.2. I have raised a feedback for this issue returning, with Apple. It appears to be a more involved issue, that will take sometime to resolve.