Another way to synchronize your calendars and contacts is using the free plugin CalDav Synchronizer.

You can download it directly here: https://caldavsynchronizer.org/en/download/

After installation a new ribbon "CalDav Synchronizer" becomes available in Outlook. Please choose "Synchronization Profiles" to add a new profile.


Click on marked symbol to define the new profiles.


Choose "mailbox.org" afterwards:


Proceed with the configuration...

1: First, type in Your main address.
2: Then check "Use IMAP/Pop3 account password".
Click the buttons number 3 and 4 in that order.


Do the next steps for every tab:


Click onto the three dots for the ressource, You want to synchronize. Assign it to an appropriate folder or create one:


Check, that You did not forget something and click "OK":