LocomotiveCMS

I was trying to get the i18n-branch to work, but no matter what I try, I keep getting this error.
The last part of the stacktrace tells me, that the problem is coming from app/controllers/admin/base_controller.rb:80:in 'set_site_locale'
When I check the mongodb-console, i see that the db gets created but is absolutely empty. When I try the same thing on the master-branch, everything works like a charm.
Could anybody point me in the direction what I am missing here? Any help would be very much appreciated.

with best regards
mandi