The calendar in the latest version of Ubuntu (18.04) (not Thunderbird!) can be synchronized with various online sources.
Including Microsoft Exchange, Next Cloud and Google.
I’ve tried the Exchange one as that usually works on mobile phones. But it didn’t. Ubuntu says can not find WebDav endpoint.
Is there any way to synchronize the Kopano calendar with the Ubuntu calendar (not Thunderbird!).
Not sure if this makes any difference but we have the paid version of Kopano.