Can't add featured image

Some time between the 25th and the 27th of July, my “Select featured image” and “Use as featured image” stopped working.

When I try to add or use as featured image, everything looks ok, until after I save. Then the featured image disappears from the post.

Any suggestions on what might be wrong?

The only changes I did, was to update plugins, but I can’t remember which ones, and I can’t find anything in the error logs.