500 when creating Wiki Page

When I try to create a Wiki page, I get

Started POST "/laublab/vray-bridge/wikis" for 91.52.29.2 at 2016-04-26 19:11:13 +0200
Processing by Projects::WikisController#create as HTML
Parameters: {"utf8"=>"✓", "authenticity_token"=>"[FILTERED]", "wiki"=>{"title"=>"vraybridge parameter visibility levels", "format"=>"markdown", "content"=>"Hello", "message"=>""}, "namespace_id"=>"laublab", "project_id"=>"vray-bridge"}
Completed 200 OK in 88ms (Views: 0.6ms | ActiveRecord: 5.5ms)
Completed 500 Internal Server Error in 106ms (ActiveRecord: 6.3ms)
NoMethodError (undefined method `username' for #<Group:0x00000007b65ba0>):
app/models/project_wiki.rb:158:in `default_message'
app/models/project_wiki.rb:152:in `commit_details'
app/models/project_wiki.rb:91:in `create_page'
app/models/wiki_page.rb:183:in `save'
app/models/wiki_page.rb:143:in `create'
app/services/wiki_pages/create_service.rb:6:in `execute'
app/controllers/projects/wikis_controller.rb:58:in `create'
lib/gitlab/middleware/go.rb:16:in `call'
Started GET "/admin/logs" for 91.52.29.2 at 2016-04-26 19:11:16 +0200
Processing by Admin::LogsController#show as HTML

I have no idea how to fix this or where it comes from. It might be important to note that I installed GitLab on Ubuntu 15.05 wily from the Ubuntu 14.04 trusty package source. There were no problems however! And apart from the issue mentioned above, GitLab seems to run smooth.

I don't think its important, but just for completeness: I messed up a previous GitLab installation trying to restore a backup from a different version (yes, I know that is not officially supported, that's probably why things were messed up afterwads). But I ran gitlab-rake gitlab:setup which should've created a fresh installation of GitLab (it even prompted me that all previous data will be lost).

gitlab:env:info

System information
System:         Ubuntu 15.10
Current User:   git
Using RVM:      no
Ruby Version:   2.1.8p440
Gem Version:    2.5.1
Bundler Version:1.10.6
Rake Version:   10.5.0
Sidekiq Version:4.0.1

GitLab information
Version:        8.7.0
Revision:       f2a3f70
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     postgresql
URL:            https://vray.niklasrosenstein.com
HTTP Clone URL: https://vray.niklasrosenstein.com/some-group/some-project.git
SSH Clone URL:  git@vray.niklasrosenstein.com:some-group/some-project.git
Using LDAP:     no
Using Omniauth: no

GitLab Shell
Version:        2.7.2
Repositories:   /var/opt/gitlab/git-data/repositories
Hooks:          /opt/gitlab/embedded/service/gitlab-shell/hooks/
Git:            /opt/gitlab/embedded/bin/git