I have roughly the same problem. In 2.9 and lower, I had a pop up when I uploaded an image. Now I get full screen. When I click insert into post, it says “done”, but no image is uploaded and it won’t revert to the post.
After trying everything else without success, I downloaded a fresh version of 2.91, re-installed and everything is working well.
I have a blank permalinks settings page as well. I have four WP sites, my newest one with WP 2.91 is the only problematic one. Anysuggestions would be greatly appreciated.
I tried uploading a screenshot, but it brought me to a password protected link.
