I seem to have solved the problems! Although I do not understand exactly how.
First, I removed the Week Calendar app since it did not sync. Then I added the Digical calendar. Since I also had problems with emailing from one account as the smtp server details had to be different for WIFI and for 3G (different providers). I tried the smtp.gmail.com solution, but it only worked for one account.
So I decided to only use the gmail app (with the non-gmail accounts added, also through the 'alias' method).
Somehow in that process, my outlook.com account on which I synchronise my Outlook PC application calendar got connected to the gmail app with incoming server: i-map-mail.outlook and outgoing server: smtp-mail.outlook.com port:587 and safety type: STARTTLS. For the incoming server, with verification needed (using the Microsoft account with outlook.com account).
I had almost given up on the whole calendar syncing issue, but decided to check once more (now in the Digical app) whether any synchronisation took place. And you know what: the synchronisation worked again. Now I have installed Week Calendar again and also that synchronises (very fast even).
I guess the synchronisation now goes through a connection between the outlook.com account via gmail and then to the google/Android calendar.
I am glad that it works, but still Microsoft and Google (and Samsung) should make this synchronisation easier by suppling good guidelines for how to organise this. Maybe a suggestion to Microsoft: make an app that creates the right account details at the start.
Note: I still have a Microsoft Exchange Active Sync account as in the past these proved necessary for automatically syncing contacts. Maybe this is not necessary anylonger, but also for this: a proper guideline from Google, Samsung and Microsoft on this would be heartily welcomed!
I would appreciate when other users of Android phones would share their solutions to syncing of the Android phone calendar and their Outlook PC and outlook.com calendars!