[Elasticsearch] Indexing error: control characters are not allowed at line 1 column 1 (Psych::SyntaxError)

https://gitlab.zendesk.com/agent/tickets/52755

Meanwhile we have upgraded to GITLAB 8.17.2-ee ;

We followed the instructions of https://docs.gitlab.com/ee/integration/elasticsearch.html

We wanted to start from the beginning; so we scratched the indexes and wanted to recreate them; now we get the following problem (see file-attatchment);

I, [2017-03-15T14:09:18.313041 #34752]  INFO -- : Indexing TestGroup / TestElasticSearch (ID=158)...
W, [2017-03-15T14:09:20.320975 #34752]  WARN -- : /opt/gitlab/embedded/lib/ruby/2.3.0/psych.rb:379:in `parse': (<unknown>): control characters are not allowed at line 1 column 1 (Psych::SyntaxError)
        from /opt/gitlab/embedded/lib/ruby/2.3.0/psych.rb:379:in `parse_stream'
        from /opt/gitlab/embedded/lib/ruby/2.3.0/psych.rb:327:in `parse'
        from /opt/gitlab/embedded/lib/ruby/2.3.0/psych.rb:254:in `load'
        from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/github-linguist-4.7.6/lib/linguist/samples.rb:24:in `cache'
        from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/github-linguist-4.7.6/lib/linguist/language.rb:525:in `<module:Linguist>'
        from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/github-linguist-4.7.6/lib/linguist/language.rb:17:in `<top (required)>'
        from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/github-linguist-4.7.6/lib/linguist.rb:4:in `require'
        from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/github-linguist-4.7.6/lib/linguist.rb:4:in `<top (required)>'
        from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/gitlab-elasticsearch-git-1.0.1/lib/elasticsearch/git/lite_blob.rb:1:in `require'
        from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/gitlab-elasticsearch-git-1.0.1/lib/elasticsearch/git/lite_blob.rb:1:in `<top (required)>'
        from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/gitlab-elasticsearch-git-1.0.1/lib/elasticsearch/git/repository.rb:6:in `require'
        from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/gitlab-elasticsearch-git-1.0.1/lib/elasticsearch/git/repository.rb:6:in `<top (required)>'
        from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/gitlab-elasticsearch-git-1.0.1/lib/elasticsearch/git.rb:3:in `require'
        from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/gitlab-elasticsearch-git-1.0.1/lib/elasticsearch/git.rb:3:in `<top (required)>'
        from /opt/gitlab/embedded/service/gitlab-rails/bin/elastic_repo_indexer:6:in `require'
        from /opt/gitlab/embedded/service/gitlab-rails/bin/elastic_repo_indexer:6:in `<main>'
indexing TestGroup / TestElasticSearch (ID=158), trace - ["/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/elastic/indexer.rb:61:in `run_indexer!'", "/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/elastic/indexer.rb:39:in `run'", "/opt/gitlab/embedded/service/gitlab-rails/app/workers/elastic_batch_project_indexer_worker.rb:25:in `run_indexer'", "/opt/gitlab/embedded/service/gitlab-rails/app/workers/elastic_batch_project_indexer_worker.rb:16: