Ludwig can you test this.

In html/layout/navigation.html this line look wrong I don't have jinja 
installed on my pi, so I can't test, but this looks broken:

    <input type="hidden" name="@action" value="{{ 
i18n.gettext('Login')|u }}">

Can you change this line (should be at line 142) to read:

    <input type="hidden" name="@action" value="Login">

there is no sense in translating a hidden field.

If this fixes things, I'll do a search and destroy for the use of 
i18n.gettext in a hidden field. E.G.

  file.index.html:27:            <input type="hidden" name="@action" 
value="{{ i18n.gettext('edit')|u }}">