Second incremental Time Machine backup won't work due to insufficient space, but there is sufficient space

I set up my PR4100 with a Time Machine backup from a MacBook Pro, as shown screenshot #1. The initial backup went fine, see screenshot #2.

When it tries to do subsequent incremental Time Machine backups it produces the error show in screenshot #3 complaining that there’s in sufficient space. But there is a ton of space both allocated in the Time Machine backup and on the PR4100 as a whole there’s many TB of space available. What’s going on? Maybe it’s a goofy glitch, I could start over, but I’d hate to have to keep doing this.


You can try the t-shooting steps provided in the following article for PR4100: