The Satellite 6 upgrade fails at "step update_repository_metadata failed"

Issue

• The upgrade shows error as below and does not complete,

# katello-installer --upgrade
Upgrading...
Upgrade Step: stop_services...
Upgrade Step: start_mongo...
Upgrade Step: migrate_pulp...
Upgrade Step: start_httpd...
Upgrade Step: migrate_candlepin...
Upgrade Step: migrate_foreman...
Upgrade Step: Running installer...
Installing             Done                                               [100%] [.....................................................................................................]
  The full log is at /var/log/katello-installer/katello-installer.log
Upgrade Step: restart_services...
Upgrade Step: db_seed...
Upgrade Step: errata_import (this may take a while) ...
Upgrade Step: update_gpg_urls (this may take a while) ...
Upgrade Step: update_repository_metadata (this may take a while) ...
Upgrade step update_repository_metadata failed. Check logs for more information.
/usr/lib/ruby/gems/1.8/gems/kafo-0.6.5.9/lib/kafo/logger.rb:104:in `error': wrong number of arguments (11 for 1) (ArgumentError)
        from /usr/lib/ruby/gems/1.8/gems/kafo-0.6.5.9/lib/kafo/logger.rb:104:in `send'
        from /usr/lib/ruby/gems/1.8/gems/kafo-0.6.5.9/lib/kafo/logger.rb:104:in `dump_buffer'
        from /usr/lib/ruby/gems/1.8/gems/kafo-0.6.5.9/lib/kafo/logger.rb:104:in `each'
        from /usr/lib/ruby/gems/1.8/gems/kafo-0.6.5.9/lib/kafo/logger.rb:104:in `dump_buffer'
        from /usr/lib/ruby/gems/1.8/gems/kafo-0.6.5.9/lib/kafo/logger.rb:103:in `each'
        from /usr/lib/ruby/gems/1.8/gems/kafo-0.6.5.9/lib/kafo/logger.rb:103:in `dump_buffer'
        from /usr/lib/ruby/gems/1.8/gems/kafo-0.6.5.9/lib/kafo/logger.rb:94:in `dump_errors'
        from /usr/lib/ruby/gems/1.8/gems/kafo-0.6.5.9/lib/kafo/logger.rb:99:in `dump_errors'
        from /usr/lib/ruby/gems/1.8/gems/kafo-0.6.5.9/lib/kafo/exit_handler.rb:26:in `exit'
        from /usr/lib/ruby/gems/1.8/gems/kafo-0.6.5.9/lib/kafo/kafo_configure.rb:122:in `exit'
        from /usr/share/katello-installer/hooks/post/30-upgrade.rb:34:in `fail_and_exit'
        from /usr/share/katello-installer/hooks/post/30-upgrade.rb:28:in `upgrade_step'
        from /usr/share/katello-installer/hooks/post/30-upgrade.rb:44:in `load'
        from /usr/lib/ruby/gems/1.8/gems/kafo-0.6.5.9/lib/kafo/hook_context.rb:13:in `instance_exec'
        from /usr/lib/ruby/gems/1.8/gems/kafo-0.6.5.9/lib/kafo/hook_context.rb:13:in `execute'
        from /usr/lib/ruby/gems/1.8/gems/kafo-0.6.5.9/lib/kafo/hooking.rb:49:in `execute'
        from /usr/lib/ruby/gems/1.8/gems/kafo-0.6.5.9/lib/kafo/hooking.rb:47:in `each'
        from /usr/lib/ruby/gems/1.8/gems/kafo-0.6.5.9/lib/kafo/hooking.rb:47:in `execute'
        from /usr/lib/ruby/gems/1.8/gems/kafo-0.6.5.9/lib/kafo/kafo_configure.rb:339:in `run_installation'
        from /usr/lib/ruby/gems/1.8/gems/kafo-0.6.5.9/lib/kafo/exit_handler.rb:24:in `call'
        from /usr/lib/ruby/gems/1.8/gems/kafo-0.6.5.9/lib/kafo/exit_handler.rb:24:in `exit'
        from /usr/lib/ruby/gems/1.8/gems/kafo-0.6.5.9/lib/kafo/kafo_configure.rb:122:in `exit'
        from /usr/lib/ruby/gems/1.8/gems/kafo-0.6.5.9/lib/kafo/kafo_configure.rb:338:in `run_installation'
        from /usr/lib/ruby/gems/1.8/gems/kafo-0.6.5.9/lib/kafo/kafo_configure.rb:109:in `execute'
        from /usr/lib/ruby/gems/1.8/gems/clamp-0.6.2/lib/clamp/command.rb:67:in `run'
        from /usr/lib/ruby/gems/1.8/gems/clamp-0.6.2/lib/clamp/command.rb:125:in `run'
        from /usr/lib/ruby/gems/1.8/gems/kafo-0.6.5.9/lib/kafo/kafo_configure.rb:116:in `run'
        from /usr/sbin/katello-installer:46

• The /var/log/katello-installer/katello-installer.log shows error as below,

[ INFO 2016-02-26 08:09:24 main] Upgrade Step: update_repository_metadata (this may take a while) ...
[ERROR 2016-02-26 08:10:00 main] rake aborted!
Required lock is already taken by other running tasks.
Please inspect their state, fix their errors and resume them.

Required lock: read
Conflicts with tasks:
- https://satellite.example.com/foreman_tasks/tasks/fcef4b79-8f83-4f57-ad10-d9167d59599b