`hammer user-group external create` command fails with a `Could not create external user group` error

Issue

Creating an external user group using the following command fails:

# hammer user-group external create --location-id <location_id> --organization-id <organization_id> --name <external_usergroup_name> --user-group <usergroup_name> --auth-source-id <id_of_linked_authentication_source>

Could not create external user group:
  Internal Server Error: the server was unable to finish the request. This may be caused by unavailability of some required service, incorrect API call or a server-side bug. There may be more information in the server's logs.