Missing Push Events after Merge Request has been accepted

Sometimes, after a Merge Request has been accepted, the according Push Event for this MR is not shown in the Activity View and even worse: the web hook will not be triggered and so our CI environment is not notified. The following screenshot shows a single example from today where this happened (the green markings are successful merges + pushes, the red one is only showing the merge event but not the push):

In the end, the MR really has been merged with the master branch, so it's not like this hasn't been executed properly.

And I don't know if this is really related, but in this timeframe the following error has been logged:

…tivesupport-4.2.5.2/lib/active_support/dependencies.rb:  492:in `load_missing_constant'
…tivesupport-4.2.5.2/lib/active_support/dependencies.rb:  184:in `const_missing'
…b-rails/lib/banzai/pipeline/plain_markdown_pipeline.rb:    6:in `filters'
…/gitlab-rails/lib/banzai/pipeline/combined_pipeline.rb:   13:in `each'
…/gitlab-rails/lib/banzai/pipeline/combined_pipeline.rb:   13:in `flat_map'
…/gitlab-rails/lib/banzai/pipeline/combined_pipeline.rb:   13:in `filters'
…vice/gitlab-rails/lib/banzai/pipeline/base_pipeline.rb:   15:in `html_pipeline'
…vice/gitlab-rails/lib/banzai/pipeline/base_pipeline.rb:   23:in `block (2 levels) in singleton class'
…b/embedded/service/gitlab-rails/lib/banzai/renderer.rb:   31:in `render_result'
…b/embedded/service/gitlab-rails/lib/banzai/renderer.rb:   61:in `cacheless_render'
…b/embedded/service/gitlab-rails/lib/banzai/renderer.rb:   26:in `render'
…service/gitlab-rails/lib/banzai/reference_extractor.rb:   31:in `analyze'
…service/gitlab-rails/lib/gitlab/reference_extractor.rb:   17:in `analyze'
…ice/gitlab-rails/lib/gitlab/closing_issue_extractor.rb:   23:in `closed_by_message'
…lab/embedded/service/gitlab-rails/app/models/commit.rb:  174:in `closes_issues'
…/service/gitlab-rails/app/services/git_push_service.rb:  106:in `block in process_commit_messages'
…/service/gitlab-rails/app/services/git_push_service.rb:   96:in `each'
…/service/gitlab-rails/app/services/git_push_service.rb:   96:in `process_commit_messages'
…/service/gitlab-rails/app/services/git_push_service.rb:   43:in `execute'
…edded/service/gitlab-rails/app/workers/post_receive.rb:   44:in `block in perform'
…edded/service/gitlab-rails/app/workers/post_receive.rb:   28:in `each'
…edded/service/gitlab-rails/app/workers/post_receive.rb:   28:in `perform'
…ruby/2.1.0/gems/sidekiq-4.0.1/lib/sidekiq/processor.rb:  150:in `execute_job'
…ruby/2.1.0/gems/sidekiq-4.0.1/lib/sidekiq/processor.rb:  132:in `block (2 levels) in process'
…1.0/gems/sidekiq-4.0.1/lib/sidekiq/middleware/chain.rb:  127:in `block in invoke'
…b-rails/lib/gitlab/sidekiq_middleware/memory_killer.rb:   17:in `call'
…1.0/gems/sidekiq-4.0.1/lib/sidekiq/middleware/chain.rb:  129:in `block in invoke'
…4.1.311/lib/new_relic/agent/instrumentation/sidekiq.rb:   33:in `block in call'
…ic/agent/instrumentation/controller_instrumentation.rb:  362:in `perform_action_with_newrelic_trace'
…4.1.311/lib/new_relic/agent/instrumentation/sidekiq.rb:   29:in `call'
…1.0/gems/sidekiq-4.0.1/lib/sidekiq/middleware/chain.rb:  129:in `block in invoke'
…q-4.0.1/lib/sidekiq/middleware/server/active_record.rb:    6:in `call'
…1.0/gems/sidekiq-4.0.1/lib/sidekiq/middleware/chain.rb:  129:in `block in invoke'
…ekiq-4.0.1/lib/sidekiq/middleware/server/retry_jobs.rb:   74:in `call'
…1.0/gems/sidekiq-4.0.1/lib/sidekiq/middleware/chain.rb:  129:in `block in invoke'
…sidekiq-4.0.1/lib/sidekiq/middleware/server/logging.rb:   11:in `block in call'
…m/ruby/2.1.0/gems/sidekiq-4.0.1/lib/sidekiq/logging.rb:   30:in `with_context'
…sidekiq-4.0.1/lib/sidekiq/middleware/server/logging.rb:    7:in `call'
…1.0/gems/sidekiq-4.0.1/lib/sidekiq/middleware/chain.rb:  129:in `block in invoke'
…1.0/gems/sidekiq-4.0.1/lib/sidekiq/middleware/chain.rb:  132:in `call'
…1.0/gems/sidekiq-4.0.1/lib/sidekiq/middleware/chain.rb:  132:in `invoke'
…ruby/2.1.0/gems/sidekiq-4.0.1/lib/sidekiq/processor.rb:  127:in `block in process'
…ruby/2.1.0/gems/sidekiq-4.0.1/lib/sidekiq/processor.rb:  166:in `stats'
…ruby/2.1.0/gems/sidekiq-4.0.1/lib/sidekiq/processor.rb:  126:in `process'
…ruby/2.1.0/gems/sidekiq-4.0.1/lib/sidekiq/processor.rb:   79:in `process_one'
…ruby/2.1.0/gems/sidekiq-4.0.1/lib/sidekiq/processor.rb:   67:in `run'
…/gem/ruby/2.1.0/gems/sidekiq-4.0.1/lib/sidekiq/util.rb:   16:in `watchdog'
…/gem/ruby/2.1.0/gems/sidekiq-4.0.1/lib/sidekiq/util.rb:   24:in `block in safe_thread'

The version we currently use is 8.5.8-ee using the pre-configured Docker container. If you need more information about our setup please ask! I try to gather anything that might be relevant for this!