Since 2017, you can no longer connect iCloud to your normal data. Now you need to create an application-specific password for all applications that want to use your iCloud calendar. Do the following:
- Go to the following address and log in with your Apple iCloud data: https://appleid.apple.com/account/manage
- In the overview you can see all application specific passwords under "Login and Security" at the bottom right tile "Application Specific...". Click on "Create password" at "Application specific passwords".
- Give the password a name e.g. "Calenso"
- Now you will be shown code in the format XXXX-XXXX-XXXX-XXXX. Copy this code and paste it in step 5 of this guide.