Can not confirm meeting invitations

Hello all,

I have an interesting effect: When I receive an invitation to an meeting I can not confirm this.
From the beginning: I am working with Thunderbird 68.10.0 under Ubunutu, my calenders are hosted on nextcloud 19.
When I receive an invitation via mail in Thunderbird I can click on “accept” it seems that nothing happens, but the meeting is added to Thunderbird (unconfirmed) and synced to the nextcloud calender. But no confirmation is send.
My mail account is a google business account, all google calendars are empty since I host my data on nextcloud. Beside the meetings I am invited to. These are also (or primarily?) placed in the google calendar and from here I can confirm the meeting.
So what am I missing ? What do I need to change to get confirmations to work out of Thunderbird / Nextcloud calender?

Thanks a lot!