Just putting this out there, primarily wondering whether this seems to be a common problem, and secondarily if anyone of you guys here has any additional tips for troubleshooting beyond what I've already done. The problem I've been running Time Machine backups to my Linux home server (currently running Ubuntu 16.04) for as long as I've had Macs. The solution is based on netatalk and avahi-daemon and has mostly been robust as long as the actual network connections have been good. Yesterday I rebooted my MacBook Pro (late 2013 retina), probably for the first time since installing Sierra, and my backups stopped working. I've had a few error messages along the way, but the short of it is this: "The network backup disk could not be accessed because there was a problem with the network username or password. You may need to re-select the backup disk and enter the correct username and password." Troubleshooting steps - When I connect to the share I use for TimeMachine using Finder, or, after mounting it, using Terminal, I can create directories and files on it. - I created a second AFP share which I too can access for both reading and writing, but Time Machine keeps repeating that the credentials are wrong. - I deleted all credentials related to anything AFP:// in my Keychain and recreated a connection, and that did not solve the problem. - Based on (ancient) tips from Apple's support forums, I renamed the target of the AFP:// credentials, renaming "[servername].afpovertcp._tcp.local." to both "[servername].local" and to "[servername].local." This didn't solve the problem either. - Based on other tips I tried using tmutil from the terminal to manually point to my backup target. It accepted the command but the subsequent backup failed. - Attempting to switch between the two available AFP shares on the server results in a "Keychain error -25299 while creating a System Keychain entry for the username "[myuser]" and URL "afp://[username]@[servername]._afpovertcp._tcp.local./[shareName]". - Again, removing the apparently faulty credentials from the keychain and retrying to establish a connection from within Time Machine fails, but after typing them in for Time Machine to use, I still have read/write access to the actual share using the same credentials. --- Any tips for what I may be missing would be greatly appreciated.