Apple Photos - Unable to Upload

After upgrading to an 200GB iCloud storage plan the uploading routine proceeded to upload 2808 photos but failed to upload 381 photos. During the upload process the photos were removed from their respective albums but not from my photo library.

• There is plenty of iCloud storage
• The internet connection is fine
• It is more recently created photos that are more likely to be affected. Though, some recent photos have uploaded successfully.
• jpeg and Nikon NEF (raw) pictures can both potentially be impacted
• Brand new pictures imported to Apple Photos (as opposed to the ones already on Apple Photos) upload successfully

The following steps did not resolve the issue:

• Upgrading the Mac OS software to the latest version
• Exporting the unmodified version photos to a separate folder on my Mac and re-importing to Apple Photos
• Moving an individual photo from "Unable to Upload" to either an existing or brand new album
• Attempting an Apple Photo library fix

Any ideas?