rdbf i have same server for app-db and no email service in my cluster, and had same problems, so email servers aren’t the cause. I created a new backup server EXT4 and, NOW, it seems it works, but for the first days it did only full backups, even the new server. In addition, every backup role movement for every website create a x3/x4 disk usage on target server. With live monitor, i saw usage down from source (2gb) and up in the target (for 10gb), so i think that the problem was in the hard links management and coding process.
With 12.0.22 and the fix permission command i have to say that, for the moment, latest backups are incremental and restore works as expected.
But I have to wait few days cause the rotation should clean the old bugged full backups, I went from 250GB to 900GB actually 😃