Encoding::InvalidByteSequenceError: Problem with german umlauts

When pushing a new branch that contains an "Umlaut", Gitlab throws an exception:

remote: /opt/gitlab/embedded/lib/ruby/2.1.0/json/common.rb:223:in `encode': "\xC3" on US-ASCII (Encoding::InvalidByteSequenceError)
remote:         from /opt/gitlab/embedded/lib/ruby/2.1.0/json/common.rb:223:in `generate'
remote:         from /opt/gitlab/embedded/lib/ruby/2.1.0/json/common.rb:223:in `generate'
remote:         from /opt/gitlab/embedded/lib/ruby/2.1.0/json/common.rb:394:in `dump'
remote:         from /opt/gitlab/embedded/service/gitlab-shell/lib/gitlab_post_receive.rb:25:in `update_redis'
remote:         from /opt/gitlab/embedded/service/gitlab-shell/lib/gitlab_post_receive.rb:18:in `exec'
remote:         from hooks/post-receive:17:in `<main>'

To reproduce the issue, create a new branch (e.g. "Änderung") and push it to the Gitlab server.

The new branch also doesn't show up in the project overview.

We are using Gitlab 7.5.1, installed via omnibus.