When libecryptfs0/ecryptfs-utils are installed, users-admin displays a
checkbox when adding a user:

  Encrypt home folder to protect sensitive data

When this checkbox is selected, attempting to add the user results in the
following error dialog:

  The configuration could not be saved
  An unknown error occurred.

The only notable console output that may correlate with this error:

  (users-admin:9021): Liboobs-WARNING **: There was an unknown error
  communicating with the backends: Message did not receive a reply (timeout
  by message bus)