Can download but cannot upload individual files to NightHawk external USB storage via ReadyCLOUD

I have two Netgear NightHawk X4S R7800 routers running firmware V1.0.2.90. I connected a 1 TB exteranl USB hard drive to one and a 512 GB external hard drive to the other. Both hard drives have single NTFS partitions. Both have ReadySHARE storage registered with ReadyCLOUD using two different accounts. I can read and write files and folders from the UNC path of both routers on my local network with either Windows 10 or Mac OS.

Issue on both: I can read files but cannot write files via ReadyCLOUD.netgear.com. File size doesn't matter. I've tried drag and drop and using the upload icon. Each time I try to uplaod a file I get the error:

"Failed to uplaod 'filename.***' to \\RouterName\tmp\mnt\sda1\FolderName..."
Connection problem Elapsed: 0:00:01 Left: #:##:##

I've tried Edge, Chrome and Safari both on and off the network.

While logged in to ReadyCLOUD I can create new folders and I can read and download files. Uploading files via ReadyCLOUD is the only thing that doesn't work. The ReadyCLOUD login page has a valid certificate issued by Amazon. As soon as I log in however, my browser reports that the site (showing my router) doesn't have a certificate at all. I'm re-directed from HTTPS to HTTP with no encryption. This is not a NAS so RAIDar is not applicable and the firmware is current as of this post. Anybody have any ideas on why I can't upload files with ReadyCLOUD?