The Bug is somewhere else 
and existing since some time.
Inside lib/python3/cmk/base/checkers/agent.py the system checks for the agent version. If you have now a special agent who outputs something like the actual Fritzbox an OS value but no version this code checks if the version field as a specific content.
And here is the problem it checks for the wrong values. Only for “(unknown)” and “None” but the actual output is “unknown”.
Quick solution → edit the file
Line 295 to
if agent_version in ['(unknown)', 'None', 'unknown']:
I will make a small pull request. This check only happens if an OS value is present.
Short solution without modify the code → remove non agent hosts from the version check - as this ones are usually skipped
EDIT: PR done