Time Machine to a Synology network share stopped working after Monterey

I had a Time Machine backing up to a Synology share mounted on my Macbook running OSX12.6 .This was working perfectly, right up until around the time I upgrade to Monterey. Apparently it started to fail silently in the background until I noticed and tried to force it to backup and started getting the error message: "Time Machine couldn't back up to.. "

This is unusual because I can mount the share, and see the sparsefile, I can even verify it in Disk Utility. It's also listed as an available disk in the TM UI. I tried some experiments with re-naming the sparsefile to see if it would work if it had to re-create the file, and it renamed the old one and still failed with an Error 35.

So, it's not a network connection problem. It's not a file access problem, since it can write to the file just fine. The backup isn't corrupt. Time Machine just doesn't seem to be able to back up to it.. any ideas?

Connecting over SMB.