Time Machine error code 21 and 19, "Failed to mount.... because the image could not be attached"
This turned out to be a keychain problem. Possibly I changed the NAS login password and never updated keychain.


Settings > Time Machine > Stop using "TimeMachine" for Backup
Keychain > search for NAS hostname and delete all matching passwords. I think the problem entries were in the System keychain, but I also deleted entries from the login keychain.
Settings > Time Machine > Add Backup Disk... > select "TimeMachine" > enter password


This situation was confusing because the messages from Time Machine didn't say anything about authentication, and Finder could connect to the NAS volume ok. But apparently backupd uses a different Keychain entry, which was bad or wrong.