I’ve been using Samba with the time machine setting for years to back up the couple of Macs I own. I’ve recently been running into issues with Time Machine backups erroring out with "The network backup disk does not support the required capabilities". Poking around the Internet didn’t really point at an obvious culprit until I found some mumblings about this potentially begin a disk space issue.

Bingo!

While the Samba server had more than enough disk space left, I had set the maximum Time Machine volume size to 3.5TB using the setting fruit:time machine max size = 3.5T. Bumping this up to 4.5T magically made Time Maching backups work immediatly.