Talk crashing when more than 2 Users

We´r using Nextcloud behind reverse proxy and are using a TURN-server for Talk. It´s working fine, when only 2 users are using Talk. When 3rd user tries to join the conference, the performance of Nextcloud is going down, the Talk session is crashing an the whole Nextcloud-system is very slow. Checking server with htop shows, that system ressources are not effected, CPU is about 10% and thers´s a lot of free memory.
We´re using Uuntu 18.04 LTS and nginx.
Any ideas?