I'm having trouble setting up TimeMachine backup on a MBP Retina running 10.8.2.

The server is running Ubuntu 10.04.4 LTS with Netatalk 2.2.1. I'm currently using Netatalk to share some files over AFP, and that works great, both from Mountain Lion and from Lion. However, when I try to setup TimeMachine on any of my machines (one running Lion, the other one Mountain Lion); neither setup works, but with mixed results.

On Lion, I am able to select the "TimeMachine" volume in system preferences, but when the initial backup is about to start it failed with an error telling be the remote volume doens't support some AFP-features (which usually would mean I need to use uams_dhx2 instead of uams_dhx, but I have already done this).

On Mountain Lion, I'm not even able to select the volume within system preferences. It doens't show up.

I have added the TMShowUnsupportedNetworkVolumes in the com.apple.systempreferences.plist on both machines by executing
defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1
I even tried deleting the com.apple.systempreferences.plist file to force System Preferences to generate a new one, which I then added the entry to once more (that's why the plist in the screenshot is so short).

This is my configuration on the server:

- -ipaddr -tcp -noddp -uamlist uams_dhx.so,uams_dhx2_passwd.so -nosavepassword
~/TimeMachine		"TimeMachine"	allow:oliver cnidscheme:dbd options:usedots,upriv,tm volsizelimit:280000
Can you see something wrong in my configuration? I just can't figure this one out. I appreciate all help I can get!