Time Machine cannot connect to NAS

[engineerdan]

I've been using my TS-420 for Time Machine backups for years. However, when I updated to firmware 4.2.4, my two Macs stopped being able to connect - the Time Machine control panel can see the NAS an an available target, allows it to be selected and prompts to enter the username and password, but sits at 'Connecting to "QNAP(TimeMachine)"' then eventually times out. The Macs are running macOS sierra 10.12.3.

I've tried:
- Upgrading to the latest beta firmware 4.3.3
- Rebooting the Mac and the NAS
- Changing the Time Machine password on the NAS
- Deleting all saved passwords associated with the QNAP from the Mac's Keychain
- Confirmed that I can connect to the NAS using AFP and a different username/password

I'm tearing my hair out! How did I get Time Machine talking to the QNAP again?