missing interpolation argument :benutzername for german translation

i just registered a user via https://unstable.bitmask.net/signup, and am redirected to https://unstable.bitmask.net/de/users/d6733ad79e36dd112a3656a98f342b43 afterwards, which fails:

Jul 15 09:35:25 donkey webapp[31992]: Started GET "/de/users/d6733ad79e36dd112a3656a98f342b43" for 0.0.0.0 at 2015-07-15 09:35:25 +0000
Jul 15 09:35:25 donkey webapp[31992]: Processing by UsersController#show as HTML
Jul 15 09:35:25 donkey webapp[31992]: Parameters: {"locale"=>"de", "id"=>"d6733ad79e36dd112a3656a98f342b43"}
Jul 15 09:35:25 donkey webapp[31992]: Rendered users/_overview.html.haml (7.7ms)
Jul 15 09:35:25 donkey webapp[31992]: Rendered users/show.html.haml within layouts/application (12.9ms)
Jul 15 09:35:25 donkey webapp[31992]: Completed 500 Internal Server Error in 41.8ms
Jul 15 09:35:25 donkey webapp[31992]: I18n::MissingInterpolationArgument (missing interpolation argument :benutzername in "Willkommen %{benutzername}." ({:username=>"test565"} given)):#012  app/views/users/_overview.html.haml:3:in `_app_views_users__overview_html_haml___3116455300471431398_21214880'#012  app/views/users/show.html.haml:1:in `_app_views_users_show_html_haml__2975516538299042513_34891060'
Jul 15 09:35:25 donkey webapp[31992]: Processing by ErrorsController#server_error as HTML
Jul 15 09:35:25 donkey webapp[31992]: Parameters: {"locale"=>"de", "id"=>"d6733ad79e36dd112a3656a98f342b43"}
Jul 15 09:35:25 donkey webapp[31992]: Rendered errors/server_error.html.haml within layouts/application (4.0ms)
Jul 15 09:35:25 donkey webapp[31992]: Rendered layouts/_masthead.html.haml (2.9ms)
Jul 15 09:35:25 donkey webapp[31992]: Rendered layouts/_header.html.haml (2.8ms)
Jul 15 09:35:25 donkey webapp[31992]: Rendered layouts/_navigation.html.haml (7.2ms)
Jul 15 09:35:25 donkey webapp[31992]: Rendered layouts/_content.html.haml (13.0ms)
Jul 15 09:35:25 donkey webapp[31992]: Completed 404 Not Found in 45.2ms

(from redmine: created on 2015-07-15, closed on 2015-07-21)