"No JSON object could be decoded" or JSONDecodeError when registering a system to Satellite6

Issue

• Registering a RHEL8 system to Satellite6 fails with json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0).

Traceback (most recent call last):
  File "/usr/sbin/subscription-manager", line 11, in <module>
    load_entry_point('subscription-manager==1.23.8', 'console_scripts', 'subscription-manager')()
  File "/usr/lib64/python3.6/site-packages/subscription_manager/scripts/subscription_manager.py", line 85, in main
    return managercli.ManagerCLI().main()
  File "/usr/lib64/python3.6/site-packages/subscription_manager/managercli.py", line 2918, in main
    ret = CLI.main(self)
  File "/usr/lib64/python3.6/site-packages/subscription_manager/cli.py", line 183, in main
    return cmd.main()
  File "/usr/lib64/python3.6/site-packages/subscription_manager/managercli.py", line 506, in main
    return_code = self._do_command()
  File "/usr/lib64/python3.6/site-packages/subscription_manager/managercli.py", line 1400, in _do_command
    self.entcertlib.update()
  File "/usr/lib64/python3.6/site-packages/subscription_manager/certlib.py", line 32, in update
    self.report = self.locker.run(self._do_update)
  File "/usr/lib64/python3.6/site-packages/subscription_manager/certlib.py", line 18, in run
    return action()
  File "/usr/lib64/python3.6/site-packages/subscription_manager/entcertlib.py", line 42, in _do_update
    return action.perform()
  File "/usr/lib64/python3.6/site-packages/subscription_manager/entcertlib.py", line 141, in perform
    self.content_access_hook()
  File "/usr/lib64/python3.6/site-packages/subscription_manager/entcertlib.py", line 194, in content_access_hook
    update_data = self.content_access_cache.check_for_update()
  File "/usr/lib64/python3.6/site-packages/subscription_manager/cache.py", line 685, in check_for_update
    data = json.loads(self.read())
  File "/usr/lib64/python3.6/json/__init__.py", line 354, in loads
    return _default_decoder.decode(s)
  File "/usr/lib64/python3.6/json/decoder.py", line 339, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib64/python3.6/json/decoder.py", line 357, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

• Registering a RHEL7 system to Satellite6 fails with No JSON object could be decoded.

# subscription-manager register --org Example --activationkey production-rhel7
The system has been registered with ID: d7aa87e7-c340-458e-aa48-dfb54b8c0632
The registered system name is: server01.example.com
Traceback (most recent call last):
  File "/sbin/subscription-manager", line 9, in <module>
    load_entry_point('subscription-manager==1.24.13', 'console_scripts', 'subscription-manager')()
  File "/usr/lib64/python2.7/site-packages/subscription_manager/scripts/subscription_manager.py", line 85, in main
    return managercli.ManagerCLI().main()
  File "/usr/lib64/python2.7/site-packages/subscription_manager/managercli.py", line 2922, in main
    ret = CLI.main(self)
  File "/usr/lib64/python2.7/site-packages/subscription_manager/cli.py", line 183, in main
    return cmd.main()
  File "/usr/lib64/python2.7/site-packages/subscription_manager/managercli.py", line 506, in main
    return_code = self._do_command()
  File "/usr/lib64/python2.7/site-packages/subscription_manager/managercli.py", line 1400, in _do_command
    self.entcertlib.update()
  File "/usr/lib64/python2.7/site-packages/subscription_manager/certlib.py", line 32, in update
    self.report = self.locker.run(self._do_update)
  File "/usr/lib64/python2.7/site-packages/subscription_manager/certlib.py", line 18, in run
    return action()
  File "/usr/lib64/python2.7/site-packages/subscription_manager/entcertlib.py", line 42, in _do_update
    return action.perform()
  File "/usr/lib64/python2.7/site-packages/subscription_manager/entcertlib.py", line 141, in perform
    self.content_access_hook()
  File "/usr/lib64/python2.7/site-packages/subscription_manager/entcertlib.py", line 194, in content_access_hook
    update_data = self.content_access_cache.check_for_update()
  File "/usr/lib64/python2.7/site-packages/subscription_manager/cache.py", line 684, in check_for_update
    data = json.loads(self.read())
  File "/usr/lib64/python2.7/json/__init__.py", line 338, in loads
    return _default_decoder.decode(s)
  File "/usr/lib64/python2.7/json/decoder.py", line 366, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib64/python2.7/json/decoder.py", line 384, in raw_decode
    raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded