Since 16 Jun 2017 two-step verification is needed that third party apps like mine will work.
Please check if two-step verification is activated.
To check this please visit https://support.apple.com/de-de/HT204152 and follow the steps under “How do I set up two-step verification?”
If two-step verification is activated please set up an “app-specific” password for the app.
To create one please follow the steps on https://support.apple.com/de-de/HT204397.
Then use this app-specific password and your Apple-ID for iCloud login in my app. Please write the password exactly as it is written on the iClout website including the “-” between the characters.

Posted in: CalDAV