Hi Carsten,
There are numerous discussions in this forum discussion how to get nextcloud talk to work.
this article here can help Nextcloud 19 Released - #21 by oneitonitram
The only thing missing is the High perfomance Backend. which youd have to isntall on a separate server, as am not sure if anyone in this group uses nextcloud talk for a large meeting
Or go through this link here, will be insightfull Testing and configuring Nextcloud Talk - Support - NethServer Community