yum/dnf exits with "Error: Failed to synchronize cache for repo 'xxx'" without further details

Issue

• Executing a yum or dnf command fails in error, as in the example below

  # dnf repolist
  [...]
  Error: Failed to synchronize cache for repo 'XXX'
  

• Log file /var/log/dnf.log shows a Python backtrace

  Traceback (most recent call last):
    File "/usr/lib/python3.6/site-packages/dnf/repo.py", line 566, in load
      ret = self._repo.load()
    File "/usr/lib64/python3.6/site-packages/libdnf/repo.py", line 503, in load
      return _repo.Repo_load(self)
  RuntimeError: Failed to synchronize cache for repo 'XXX'
  
  During handling of the above exception, another exception occurred:
  
  Traceback (most recent call last):
    File "/usr/lib/python3.6/site-packages/dnf/cli/main.py", line 64, in main
      return _main(base, args, cli_class, option_parser_class)
    File "/usr/lib/python3.6/site-packages/dnf/cli/main.py", line 99, in _main
      return cli_run(cli, base)
    File "/usr/lib/python3.6/site-packages/dnf/cli/main.py", line 115, in cli_run
      cli.run()
    File "/usr/lib/python3.6/site-packages/dnf/cli/cli.py", line 1124, in run
      self._process_demands()
    File "/usr/lib/python3.6/site-packages/dnf/cli/cli.py", line 828, in _process_demands
      load_available_repos=self.demands.available_repos)
    File "/usr/lib/python3.6/site-packages/dnf/base.py", line 400, in fill_sack
      self._add_repo_to_sack(r)
    File "/usr/lib/python3.6/site-packages/dnf/base.py", line 135, in _add_repo_to_sack
      repo.load()
    File "/usr/lib/python3.6/site-packages/dnf/repo.py", line 568, in load
      raise dnf.exceptions.RepoError(str(e))
  dnf.exceptions.RepoError: Failed to synchronize cache for repo 'XXX'
  2019-05-24T09:45:02Z CRITICAL Error: Failed to synchronize cache for repo 'XXX'
  

• yum/dnf commands are failing with error Peer certificate cannot be authenticated with given CA certificates:-

  # yum update
  Updating Subscription Management repositories.
  rhel-8-for-x86_64-appstream-rpms   0.0  B/s |   0  B     00:00
  Errors during downloading metadata for repository 'rhel-8-for-x86_64-appstream-rpms':
  
    - Curl error (60): Peer certificate cannot be authenticated with given CA certificates for https://cdn.redhat.com/content/dist/rhel8/8/x86_64/appstream/os/repodata/repomd.xml [SSL certificate problem: self signed certificate in certificate chain]
  
  Error: Failed to download metadata for repo 'rhel-8-for-x86_64-appstream-rpms': Cannot download repomd.xml: Cannot download repodata/repomd.xml: All mirrors were tried