The access to an access limit folder by the AFP is not possible. Time Machine backup is not possible
Firmware update for improvement is planned.

My workaround is to mount it via SMB.


[Update] See also (in german): Praxistipps zu Lion

[Update 2] Here is a workaround on the support pages from Apple. 

Lion and Lion Server maintains a list of authentication methods that are not allowed. These are the older, less secure authentication methods. You may need to enable one or more of these methods to support legacy devices or protocols.

  1. Open Terminal.
  2. Execute the following commands:
    sudo chmod o+w /Library/Preferences
    sudo defaults write /Library/Preferences/ afp_host_prefs_version -int 1

  3. Restart the computer.
  4. For an AFP connection, on the client make an AFP connection to another system so that the AFP Client preference file will be filled in with the default set of values.If you want to enable a different User Authentication Method (UAM), execute the apprpriate Terminal command on the client system. For example, this command enables DHCAST128 by removing it from the list of disabled methods:
    sudo defaults write /Library/Preferences/ afp_disabled_uams -array "Cleartxt Passwrd" "MS2.0" "2-Way Randnum exchange"

Additional Information

To see a list of the disabled User Authentication Methods (UAM), run the following command in Terminal on the client:


defaults read /Library/Preferences/ afp_disabled_uams

Typically, the disabled UAMs  are “Cleartxt Passwrd”, “MS2.0”, “2-Way Randnum exchange”, and DHCAST128.

To enable one of these UAMs, the list of disallowed methods needs to not list the UAM that you want to exclude.

[Update3] Buffalo released some beta firmware updates. See Compatible devices