Why does GitLab merge request returns "500 Internal Server Error"?

My user wants to merge a request on our hosted GitLab EE 11.5.2 website. But the website returns 500 Internal Server Error as follows:

The related log is as follows:

Completed 500 Internal Server Error in 56565ms (ActiveRecord: 746.1ms | Elasticsearch: 0.0ms)

ActionView::Template::Error (4:Deadline Exceeded):
  42:             %li.diffs-tab
  44:                 Changes
  45:                 %span.badge.badge-pill= @merge_request.diff_size
  46:
  47:     #diff-notes-app.tab-content
  48:       #new.commits.tab-pane.active
lib/gitlab/gitaly_client/diff_stitcher.rb:13:in `each'
lib/gitlab/gitaly_client/diff_stitcher.rb:13:in `each'
lib/gitlab/git/diff_collection.rb:107:in `each_gitaly_patch'
lib/gitlab/git/diff_collection.rb:47:in `each'
lib/gitlab/git/diff_collection.rb:83:in `each_with_index'
lib/gitlab/git/diff_collection.rb:83:in `decorate!'
lib/gitlab/diff/file_collection/base.rb:32:in `diff_files'
lib/gitlab/diff/file_collection/base.rb:9:in `real_size'
app/models/merge_request.rb:411:in `diff_size'
app/views/projects/merge_requests/creations/_new_submit.html.haml:45:in `block in _app_views_projects_merge_requests_creations__new_submit_html_haml__4084413277917201848_70167460469980'
app/views/projects/merge_requests/creations/_new_submit.html.haml:43:in `_app_views_projects_merge_requests_creations__new_submit_html_haml__4084413277917201848_70167460469980'
app/views/projects/merge_requests/creations/new.html.haml:6:in `_app_views_projects_merge_requests_creations_new_html_haml__4204914316377908944_70167458972060'
app/controllers/application_controller.rb:116:in `render'
lib/gitlab/i18n.rb:55:in `with_locale'
lib/gitlab/i18n.rb:61:in `with_user_locale'
app/controllers/application_controller.rb:427:in `set_locale'
lib/gitlab/middleware/multipart.rb:101:in `call'
lib/gitlab/request_profiler/middleware.rb:14:in `call'
ee/lib/gitlab/jira/middleware.rb:15:in `call'
lib/gitlab/middleware/go.rb:17:in `call'
lib/gitlab/etag_caching/middleware.rb:11:in `call'
lib/gitlab/middleware/rails_queue_duration.rb:22:in `call'
lib/gitlab/metrics/rack_middleware.rb:15:in `block in call'
lib/gitlab/metrics/transaction.rb:53:in `run'
lib/gitlab/metrics/rack_middleware.rb:15:in `call'
lib/gitlab/middleware/read_only/controller.rb:40:in `call'
lib/gitlab/middleware/read_only.rb:16:in `call'
lib/gitlab/middleware/basic_health_check.rb:25:in `call'
lib/gitlab/request_context.rb:20:in `call'
lib/gitlab/metrics/requests_rack_middleware.rb:27:in `call'
lib/gitlab/middleware/release_env.rb:10:in

Could you please tell me how to troubleshoot this problem?

Thank you.

Eventually, we found the solution!

Though the merge request can be achieved by git command, it's not recommended by our security policy.

And the solution is that the merge request should be operated via "Merge Requests" instead of "Repository > Branches".

The successful way:

The failed way: