Android calendar app can't set notification via Email

Hi, I am using Business calendar on Android. I am syncing my calendar via DAVx5 to my nextcloud (or better vice versa). In the nextcloud calendar app I can use the email notification as alarm method for new appointments. Unfortunately my business calendar isn’t able to do that for nextcloud appointments. As soon as I switch over to the google calendar, it is possible from business calender to use email notification.

A further strange thing is, as soon as I set an email notification in the nextcloud calendar app and do a sync, the business calendar app on Android shows me the email notification but (again) it don’t allows me to set a new one.

Any hints what I can do on that?

Same issue here.
Configuring appointment with email notifications in Google app, emails nevers works