Developing a new site with Drupal 8, I was attempting to add an image to a custom block. In the editor I clicked the image icon and got a dialog to upload an image. After specifying a file (making sure it was within the upload file size), I got a strange error "Image field is required."
I took the error message literally, and created a new custom block type, adding a field of type "image" to the block. So now in the form, I have a Body and an Image field. If I try to upload using the "image" field below the body, I get the same error "Image field is required."
I tried again using the Content / Media tab, to add an image file, and get the same error:
(I also notice that there's a "Caption" checkbox but no field to enter a caption. I'm assuming I'll have a text field once the image is uploaded.)
Drupal 8.8, all current.