Steps
- Create a Booking.com source in kalenderabgleich.de.
- Copy its kalenderabgleich.de export URL.
- In Booking.com open Calendar & pricing and calendar synchronisation.
- Add the kalenderabgleich.de URL as an imported calendar.
- Copy the Booking.com export URL.
- Paste it into the source’s iCal import URL.
- Remove every other calendar link from Booking.com.
Final step: Remove all old direct calendar connections to other portals from the provider. Only the kalenderabgleich.de calendar should remain connected.