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":