Comment 1 for bug 2049785

I reproduced the bug on a fresh 20.04.6 install, just upgraded update-manager, disconnected from internet and started update manager

Failed to connect to authentication server
Check your Internet connection and try again.

[info] A new version is available: 30~20.04
Please run:
    sudo apt-get install ubuntu-advantage-tools
to get the latest version with new features and bug fixes.
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/UpdateManager/Dialogs.py", line 118, in <lambda>
    button.connect("clicked", lambda x: callback())
  File "/usr/lib/python3/dist-packages/UpdateManager/Dialogs.py", line 371, in available
    self.window_main.start_available(error_occurred=True)
  File "/usr/lib/python3/dist-packages/UpdateManager/UpdateManager.py", line 240, in start_available
    self.refresh_cache()
  File "/usr/lib/python3/dist-packages/UpdateManager/UpdateManager.py", line 458, in refresh_cache
    self._get_ua_security_status()
  File "/usr/lib/python3/dist-packages/UpdateManager/UpdateManager.py", line 284, in _get_ua_security_status
    s = json.load(p.stdout)
  File "/usr/lib/python3.8/json/__init__.py", line 293, in load
    return loads(fp.read(),
  File "/usr/lib/python3.8/json/__init__.py", line 357, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python3.8/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python3.8/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

ii ubuntu-advantage-tools 27.13.6~20.04.1 amd64 management tools for Ubuntu Pro

$ pro security-status --format=json
Failed to connect to authentication server
Check your Internet connection and try again.

[info] A new version is available: 30~20.04
Please run:
    sudo apt-get install ubuntu-advantage-tools
to get the latest version with new features and bug fixes.

that's resolved after updating ubuntu-advantage-tools to 30~20.04 (the current version)