I´m trying to fix this since more less one year. My Nextcloud goes with any update / upgrade available, but this problem wan´t go. I´m using a coturn server with it´s own public IP. It runs on 443, so it can be used from users behind corporate Firewalls. With spreed.mehttps://apps.nextcloud.com/apps/spreedme everything worked quite fine with same configuration, but why not with spreed https://apps.nextcloud.com/apps/spreed ? I have no more ideas where to tweak, look around and finally find a fix for this. Maybe there is anyone out there who had the same issue and was able to work around. Thanks in advance for your help!!!
Cheers
Update:
I guess to make stable Group Calls it´s necessary to pay for…
Spreed High Performance back-end providing MCU, STUN, TURN and SIP gate
Yes you´re right!. It´s a qemu vps with 4 cores Intel® Xeon® CPU E5-2630 v4 @ 2.20GHz, SSD and 16 GB RAM. It has an symmetric Internet connection with 100 Mbit. Regarding bandwidth it uses round about 2 - 4 Mbit per user. But it has quite an big CPU impact at the clients. If I use Firefox CPU spikes up hat 60 - 70 % while trying to make a group call with 3 users before it breaks. With Chrome it consumes half of it, but it breaks anyway. unfortunately it breaks as well if I try to connect with two Android and one iOS. I´m quite sure, it has something to do with missing MCU feature https://webrtcglossary.com/mcu/ .
I call this following idea “pass the video baton”, BTW.
The text chat functionality of Nextcloud Talk is sure to work with many participants. Use that liberally, and have that as your baseline. Then from there, get a little more courageous, and have a multiparty voice-only call (as in, turn off the video within a call by pressing the little camera button). The bandwidth and CPU demands for voice-only are pretty modest.
Then if that works smoothly enough for a group, and you all gain sufficient confidence in that, then cautiously try to add video.
Maybe have only one or two people “reveal” their video at any given time, with other participants keeping theirs turned off.
Have a “pass the baton”-style meeting where only one person at a time gets to speak (because they have the imaginary “baton”), then they “pass the baton” to the next person (and turn off their video, going audio-only) when they’re done speaking (and that next person turns their video on only when they have the baton).
Yes, this is much more fiddly, as everyone needs to understand the rules of the “pass the video baton” game and cooperate, however it would save many thousands of dollars.
Thank´s for your thoughts on this. I´m not using it for commercial purposes. For me it´s at the moment a solution to talk with my fellow students for consultation regarding a didactic-research-project. But looking a bit around in the Internet brought me to that https://jitsi.org/ . Maybe this can serve my actual needs.
Ok, today got some time to have a look again. I used mpm_event module (http2) in Apache. When I changed over to mpm_prefork, talk is able to run a stable 3 party video call . With a fourth one whole nextcloud stucks again - pressed f5 (while talk was open) in a separate admin browser session -> too slow. I´m quite a bit disappointed, due to that http2 should no problem at all with nextcloud. But why is this not solid using talk? In my opinion talk is strongly aligned for the MCU based setup, which is be sold separately and as expensive, that only enterprise users can afford it - sad! But needless to say, nextcloud in common rules, thanks !
I noticed that in the Signal Android client, the video chat button disappeared after a recent upgrade to the app. There’s only a phone icon now in a chat with any one user.
After a while, I found where the video chat feature went.
They hid it in the voice call dialog. That is to say, you first start a voice call with someone, then while talking, lo and behold, there’s the video button within the call in the upper right.
So I’m not the only one out there with this kind of “gear up gradually” mentality (first start with text, then gear up to voice, then once that’s working, then maybe gear up to video). Signal has downright designed it into their software.
I daresay Nextcloud would do well to incorporate your improvement, or something similarly empowering. Why? Because if all Nextcloud Talk can handle by default is two video callers, then why would I bother with Nextcloud Talk at all (plus the hassle of setting up coturn), when, say, Signal Private Messenger or Wire Secure Messenger already similarly offer me the ability to do 1:1 video calls.
I’m a little miffed that the install of coturn wins you so little. I too similarly have a coturn server set up (on a separate public IP address, and configured for use in Nextcloud).
. With a fourth one whole nextcloud stucks again 
- pressed f5 (while talk was open) in a separate admin browser session -> too slow. I´m quite a bit disappointed, due to that http2 should no problem at all with nextcloud. But why is this not solid using talk? In my opinion talk is strongly aligned for the MCU based setup, which is be sold separately and as expensive, that only enterprise users can afford it - sad! But needless to say, nextcloud in common rules, thanks 
!