System registration failed with "Proxy connection failed: 403"

Issue

• Subscription-manager command fails with Proxy connection failed: 403

# subscription-manager register
Network error, unable to connect to server.
Please see /var/log/rhsm/rhsm.log for more information.

• Traceback in /var/log/rhsm/rhsm.log:-

2021-07-16 18:40:33,764 [INFO]  @managercli.py:264 - Server Versions: {'candlepin': 'Unknown', 'server-type': 'This system is currently not registered.'} 
2021-07-16 18:40:33,765 [INFO]  @managercli.py:253 - Client Versions: {'python-rhsm': '1.1.8-1.el6', 'subscription-manager': '1.1.23-1.el6'} 
2021-07-16 18:40:33,765 [INFO]  @connection.py:527 - Using basic authentication as: xxx
2021-07-16 18:40:33,766 [INFO]  @connection.py:549 - Connection Built: host: subscription.rhn.redhat.com, port: 443, handler: /subscription
2021-07-16 18:40:33,766 [DEBUG]  @connection.py:360 - Loading CA PEM certificates from: /etc/rhsm/ca/
2021-07-16 18:40:33,767 [DEBUG]  @connection.py:342 - Loading CA certificate: '/etc/rhsm/ca/redhat-uep.pem'
2021-07-16 18:40:33,767 [DEBUG]  @connection.py:342 - Loading CA certificate: '/etc/rhsm/ca/candlepin-stage.pem'
2021-07-16 18:40:33,767 [DEBUG]  @connection.py:366 - Using proxy: PROXY:PORT
2021-07-16 18:40:33,768 [DEBUG]  @connection.py:381 - Making request: GET https://subscription.rhn.redhat.com:443/subscription/users/xxx/owners
2021-07-16 18:40:33,770 [ERROR]  @managercli.py:115 - Error during registration: Proxy connection failed: 403
2021-07-16 18:40:33,770 [ERROR]  @managercli.py:116 - Proxy connection failed: 403
Traceback (most recent call last):
  File "/usr/share/rhsm/subscription_manager/managercli.py", line 956, in _do_command
    owner_key = self._determine_owner_key(admin_cp)
  File "/usr/share/rhsm/subscription_manager/managercli.py", line 1057, in _determine_owner_key
    owners = cp.getOwnerList(self.username)
  File "/usr/lib64/python2.6/site-packages/rhsm/connection.py", line 697, in getOwnerList
    return self.conn.request_get(method)
  File "/usr/lib64/python2.6/site-packages/rhsm/connection.py", line 437, in request_get
    return self._request("GET", method)
  File "/usr/lib64/python2.6/site-packages/rhsm/connection.py", line 387, in _request
    conn.request(request_type, handler, body=body, headers=headers)
  File "/usr/lib64/python2.6/httplib.py", line 914, in request
    self._send_request(method, url, body, headers)
  File "/usr/lib64/python2.6/httplib.py", line 951, in _send_request
    self.endheaders()
  File "/usr/lib64/python2.6/site-packages/rhsm/connection.py", line 140, in endheaders
    httpslib.HTTPSConnection.endheaders(self)
  File "/usr/lib64/python2.6/httplib.py", line 908, in endheaders
    self._send_output()
  File "/usr/lib64/python2.6/httplib.py", line 780, in _send_output
    self.send(msg)
  File "/usr/lib64/python2.6/httplib.py", line 739, in send
    self.connect()
  File "/usr/lib64/python2.6/site-packages/M2Crypto/httpslib.py", line 203, in connect
    raise socket.error, "Proxy connection failed: %d" % code
error: Proxy connection failed: 403