I have another piece of information for you which might be relevant for the investigation. I went to “Language settings → Related settings → Administrative language settings → Language for non-Unicode programs → Change system locale” and there I ticked the checkbox named “Beta: Use Unicode UTF-8 for worldwide language support”.
After I did that and restarted PC, SAM is working as expected! No more it asks repeatedly for signing in and I have also succesfully activated my license, therefore, now I am finally able to launch Dorico 4. I would say that this observation confirms that there is a language issue involved.