Thank you for your info regarding APIs. I took your advice and delved a bit further into that. I have read that EWS APIs will be phased out and replaced by Graph Rest APIs. There is also a large but interesting discussion on this forum about the phasing out of the EWS APIs.
Could it be that this is why MS has not yet made a definitive choice for the exchange protocol with New Outlook and therefore does not want to sync with Calendar? It is also remarkable that the OneCalendat App with the new Graph Rest API protocol can sync with the longstanding W10 Mail as well as Outlook.com.
Since the Mail part of New Outlook works to my satisfaction, I will continue to use that and use the syncing versions of the Android / W10-11 Onecalendar App.
@**Umph8674. I'm using 2 MAPI mail account on New Outlook and it works without any problems. Only installing the accounts was tricky because "migration" did not include the settings from W10 Mail. After installing this manually with the provider's account settings, it works fine. I have to admit, that New Outlook offers more features than W10 Mail and that's why I keep using it, even if the calendar doesn't sync.