After updating to Sierra, Mac won't connect to ReadyNAS device
I have two Macs at home - an iMac, and a MacBook Pro. I also have a ReadyNAS device, which acts as a Time Machine for both.
I updated the iMac to macOS Sierra. The upgrade went without issue, no errors reported. However, once it was back up, I found that the iMac had issues authenticating to the ReadyNAS using AFP (and / or it would authenticate, then drop the connection), and also could not connect to the Time Machine service on the ReadyNAS device.
To be sure it was not the ReadyNAS, I went to my MBP - it's still running OS X 10.11, and is not showing any issues. AFP shares are accessible without issue, and Time Machine works as expected.
Did anything change with the AFP or Time Machine protocols in macOS Sierra, that might cause this?
The ReadyNAS Device is a ReadyNAS Pro 4, running 4.2.28 firmware, if that helps.
Thank you for your help!
iMac, macOS Sierra (10.12)