最新の安定版（2.8.10から2.8.11）にアップデートしようとしていますが、themes:updateを呼び出す際に中断されます。themes:updateタスクを手動で実行しようとしましたが、エラーは同じです。
# rake themes:update
Checking 'xyz (darker)' for 'default'... Failed to update 'xyz (darker)': undefined method `scheme' for nil:NilClass
rake aborted!
NoMethodError: undefined method `scheme' for nil:NilClass
/var/www/discourse/lib/theme_store/git_importer.rb:129:in `clone_http!'
/var/www/discourse/lib/theme_store/git_importer.rb:94:in `clone!'
/var/www/discourse/lib/theme_store/git_importer.rb:19:in `import!'
/var/www/discourse/app/models/remote_theme.rb:126:in `update_remote_version'
/var/www/discourse/lib/tasks/themes.rake:62:in `block in update_themes'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/activerecord-6.1.6.1/lib/active_record/relation/batches.rb:71:in `block (2 levels) in find_each'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/activerecord-6.1.6.1/lib/active_record/relation/batches.rb:71:in `each'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/activerecord-6.1.6.1/lib/active_record/relation/batches.rb:71:in `block in find_each'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/activerecord-6.1.6.1/lib/active_record/relation/batches.rb:138:in `block in find_in_batches'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/activerecord-6.1.6.1/lib/active_record/relation/batches.rb:245:in `block in in_batches'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/activerecord-6.1.6.1/lib/active_record/relation/batches.rb:229:in `loop'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/activerecord-6.1.6.1/lib/active_record/relation/batches.rb:229:in `in_batches'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/activerecord-6.1.6.1/lib/active_record/relation/batches.rb:137:in `find_in_batches'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/activerecord-6.1.6.1/lib/active_record/relation/batches.rb:70:in `find_each'
/var/www/discourse/lib/tasks/themes.rake:56:in `update_themes'
/var/www/discourse/lib/tasks/themes.rake:87:in `block (2 levels) in <main>'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/rails_multisite-4.0.0/lib/rails_multisite/connection_management.rb:80:in `with_connection'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/rails_multisite-4.0.0/lib/rails_multisite/connection_management.rb:90:in `each_connection'
/var/www/discourse/lib/tasks/themes.rake:86:in `block in <main>'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/bundler-2.3.22/lib/bundler/cli/exec.rb:58:in `load'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/bundler-2.3.22/lib/bundler/cli/exec.rb:58:in `kernel_load'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/bundler-2.3.22/lib/bundler/cli/exec.rb:23:in `run'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/bundler-2.3.22/lib/bundler/cli.rb:486:in `exec'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/bundler-2.3.22/lib/bundler/vendor/thor/lib/thor/command.rb:27:in `run'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/bundler-2.3.22/lib/bundler/vendor/thor/lib/thor/invocation.rb:127:in `invoke_command'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/bundler-2.3.22/lib/bundler/vendor/thor/lib/thor.rb:392:in `dispatch'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/bundler-2.3.22/lib/bundler/cli.rb:31:in `dispatch'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/bundler-2.3.22/lib/bundler/vendor/thor/lib/thor/base.rb:485:in `start'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/bundler-2.3.22/lib/bundler/cli.rb:25:in `start'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/bundler-2.3.22/exe/bundle:48:in `block in <top (required)>'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/bundler-2.3.22/lib/bundler/friendly_errors.rb:120:in `with_friendly_errors'
/var/www/discourse/vendor/bundle/ruby/2.7.0/gems/bundler-2.3.22/exe/bundle:36:in `<top (required)>'
/usr/local/bin/bundle:25:in `load'
/usr/local/bin/bundle:25:in `<main>'
Tasks: TOP => themes:update
(See full trace by running task with --trace)
問題のテーマは（1年以上）変更されておらず、実際には非常に基本的なもので、色を変更し、単一のCSSルールを追加するだけなので、Discourseで何かが変更されたようです。テーマはリモート（GitHubプライベートリポジトリ）であることは関連があるかわかりません。
その後、Webインターフェースからテーマを更新しようとすると、同様のエラーで500エラーが発生することに気づきました。
Started PUT "/admin/themes/48" for x.x.x.x at 2022-11-14 17:44:14 +0000
Processing by Admin::ThemesController#update as */*
Parameters: {"theme"=>{"remote_check"=>true}, "id"=>"48"}
Completed 500 Internal Server Error in 1904ms (ActiveRecord: 0.0ms | Allocations: 24475)
NoMethodError (undefined method `scheme' for nil:NilClass)
lib/theme_store/git_importer.rb:129:in `clone_http!'
lib/theme_store/git_importer.rb:94:in `clone!'
lib/theme_store/git_importer.rb:19:in `import!'
app/models/remote_theme.rb:126:in `update_remote_version'
app/controllers/admin/themes_controller.rb:207:in `update'
app/controllers/application_controller.rb:387:in `block in with_resolved_locale'
app/controllers/application_controller.rb:387:in `with_resolved_locale'
lib/middleware/omniauth_bypass_middleware.rb:71:in `call'
lib/content_security_policy/middleware.rb:12:in `call'
lib/middleware/anonymous_cache.rb:361:in `call'
config/initializers/100-quiet_logger.rb:23:in `call'
config/initializers/100-silence_logger.rb:31:in `call'
lib/middleware/enforce_hostname.rb:23:in `call'
lib/middleware/request_tracker.rb:198:in `call'
何を調査すべきか何かアイデアはありますか？必要であればテーマファイルを共有できます。
編集：リポジトリを公開に変更したところ、エラーが解消されたため、認証トークンを使用したプライベートリポジトリの処理に何か変更があったようです。しかし、奇妙なことに、リポジトリをプライベートに戻しても、再構築時にエラーは発生しませんでした…さらに、同じ設定（プライベートリポジトリ内）のリモートテーマが2つあり、そのうちの1つだけが失敗しました。