Time Machine "disk not available" (RN104, firmware 6.10.2)

I've been running macOS Catalina on my 2018 Mac mini since it was released, and my RN104 is on 6.10.2

Everything was fine and then I turned both Mac and NAS off while I was on vacation; since turning them back on*, I've been getting errors from Time Machine saying "Backup Disk Not Available; Try backing up again when RN104.local is available".  (*I wonder if the update to 6.10.2 happened when I rebooted, in which case it may be related to the problem - I don't recall when it updated.)

I've tested pointing Time Machine at my old NV+ and that works fine, so that (in my mind) points the finger squarely at the RN104.

I've turned off Shared Time Machine (on RN104), rebooted, re-enabled it, and I get the same error.

I've set up a Private Time Machine and get the same error.

(Incidentally, I am able to mount the shares from RN104 without any issue.)

I thought I would see if the Time Machine sparsebundle is visible on the network but I can't find it - I can, however, see the one on the NV+

This leads me to think that the RN104 isn't actually setting up for Time Machine correctly.

Any suggestions on what to check/try next?