Hello.
I humbly apologise to everyone, i realised where the problem lay.
Unfortunately, in the Windows agent registration procedure, i did not pay attention to the CHECKMK web interface.
Basically after rolling the dice to generate a new password i did not click on the Save button (i attach screenshot),
so the password i generated and used to register the agent was not saved in CHECKMK and therefore the automation user was not authenticated.
In my defence, in the documentation there is no reference to the Save button but only to the dice roll as can be seen from Users, roles and permissions - User and authorization configuration
Let me be clear, i do not want to blame the documentation; i should have been more careful.
Anyway, i thank everyone for their suggestions and apologise if i have wasted your time.
Thanks again.