I just ran into this same issue. Initially I was able to have Private shares and everything was working. As near as I can tell, this is what broke me:
I had a working share called "photos" and one called "videos". They were working fine as "private" shares. These shares were only included on "disk2" and nothing else.
I then created a new "multimedia" share that only included disk2 as well with "private" settings. It was working fine this way.
From the CLI I consolidated by doing a "mv /mnt/disk2/photos /mnt/disk2/multimedia". I then tried to mount up my multimedia share and started getting the "can't be completed" error.
I tried to reboot, move the share name, etc etc. Nothing would fix it other than setting the share to be "secure" permissions. This allows it to work just fine. Setting it back to "private" breaks it again.
Its odd to me that it was working fine, then I did the manual move operation and it started failing on ALL my shares.
I'd love for this to work with "private" again. Any thoughts?