Unable to synchronize repositories in Red Hat Satellite 6 due to sha1 or sha256 checksum errors

Issue

• Synchronizing repositories into Red Hat Satellite server fails with errors like these in /var/log/foreman/production.log:

  2020-01-01T00:00:00 [I|app|] Current user set to admin (regular)
  2020-01-01T00:00:00 [E|bac|] RPM1008: Checksum type "sha256" is not available for all units in the repository. Make sure those units have been downloaded. (Katello::Errors::PulpError)
  /opt/theforeman/tfm/root/usr/share/gems/gems/katello-3.12.0.41/app/lib/actions/pulp/abstract_async_task.rb:122:in `block in external_task='
  /opt/theforeman/tfm/root/usr/share/gems/gems/katello-3.12.0.41/app/lib/actions/pulp/abstract_async_task.rb:120:in `each'
  /opt/theforeman/tfm/root/usr/share/gems/gems/katello-3.12.0.41/app/lib/actions/pulp/abstract_async_task.rb:120:in `external_task='
  /opt/theforeman/tfm/root/usr/share/gems/gems/dynflow-1.2.3/lib/dynflow/action/polling.rb:98:in `poll_external_task_with_rescue'