I had this problem before and agonized for a week until I somehow managed to fix it, though I don’t recall what I did.
It was not my theme, nor any of my plugins that were the problem. It also wasn’t the permissions on any of the media folders as I played around with those for DAYS from one suggestion that someone had.
This has just cropped up again today. The only thing that I have done differently is that I added a user today, who hasn’t logged in and who I have already deleted.
Is there anyone from WordPress who knows how to solve this “seemingly” serious and widespread problem? ??