Some thumbnails not showing and images won’t load

Hello,

I’m having these issues:
– SOME thumbnails aren’t showing in the Media Gallery
– A new image uploaded does not show in preview.
– I can’t add newly uploaded image to post.

What I have done to diagnose the problem:
– Deactivated all plugins – Problem still present.
– Activated one plugin at a time – Found one plugin that may be causing the problem. Left plugin deactivated yet problem still exists.
– Updated all plugins – Problem persists.
– Switched to Twenty Sixteen Theme – Problem still present.
– Uploaded a new copy of the image – Problem still present.
– Created a new image and uploaded – Problem still present.
– Installed Force Regenerate Thumbnails plugin and run it – Problem still present.
– Installed Regenerate Thumbnails and run it – Problem still present.
– Opened newly uploaded image under Media and selected image, no preview showing and can’t edit image.
– Existing images that DO show are in /wp-content/uploads/YEAR/MONTH/ but new images are located in /wp-content/uploads/
Thumbnails NOT in /YEAR/MONTH do not show.
– Media – Setting Enabled – Organize my uploads into month- and year-based folders and uploaded new image – Problem still persists.

Any ideas how to fix the problem please?
Thanks very much
Dan