Windows Server 2012R2 with RDS using UPD's stopped displaying thumbnails on a network drive

Question

Windows Server 2012R2 with RDS using UPD/UVHD's doesn't display thumbnails on a network drive, well they do for a second but then revert to the file icon. We've unchecked (and checked X 100's) the "Always show icons, never thumbnails" option in Folder Options/View. We've checked (and unchecked X dozens) the "Show thumbnails instead of icons" option in System Properties/Performance Options/Visual Effects. And we've also cleared the thumbnail cache.