Nextcloud 10 and CalDavZAP setup


I am running the nextcloud server (10.0.2) without problems when using iphone or thunderbird. But since the own calendar-app seams broken (modified repeating events show up twice or get lost, …), I wanted to use a different GUI. I stumbled over CalDavZAP which has a nice GUI and seams easy to use. But I can not get it to work with nextcloud.
Has any kind soul a working config for this client ?