Time Machine failing with (null) error
I also use netatalk/afpd, backing to a USB-mounted drive.  If I partition the drive on OSX first (hfsplus, no journaling), then mount it under Linux, chmod &amp; chgrp it to the TimeMachine userID, and share with netatalk, then I get the same error you report (after "preparing backup", error is "Setting security information: Operation not permitted"), and backup fails every time.  

However, if I stop netatalk, repartition the USB drive to a single EXT4 partition, mkfs.ext4, remount and chmod, chgrp, and share with netatalk, then I do not see this error and TimeMachine backups work fine.

I expect that the problem is userID/permission related, but since ETX4 is working for me, I have not pursued it.