Issues when using Talk with 4 participants; looking for clues how to debug

We are using NC 14 with Talk 4.0.2. But we also tried it with former versions. Every time we tried it, we cannot have a decent conversation. Random participants drops out, making connections are hard, video is getting out of sync.

NC is running on a VPS, 4GB ram, 6GB swap, SSD, 100Mbit synchronous network connection, no restrictions. It’s installed on a Debian stable environment, apache 2.4.25, php-fpm 7.0. I tried adding coturn in the mix, but to no avail.

I want to debug this, but I’m looking for clues what we can do to do that. Last time we tried, I checked NC, php and apache logfiles, but couldn’t find anything related.

# sudo -u www-data php -f occ config:list spreed
{
    "apps": {
        "spreed": {
            "enabled": "yes",
            "installed_version": "4.0.2",
            "signaling_servers": "{\"servers\":[],\"secret\":\"\"}",
            "stun_servers": "[\"stun.nextcloud.com:443\"]",
            "turn_servers": "[{\"server\":\"c.nllgg.nl\",\"secret\":\"<REDACTED>\",\"protocols\":\"udp,tcp\"}]",
            "types": "prevent_group_restriction"
        }
    }
}
# turnserver.conf
fingerprint
use-auth-secret
static-auth-secret=<REDACTED>
realm=c.nllgg.nl
total-quota=100
bps-capacity=0
cert=/etc/letsencrypt/live/c.nllgg.nl/cert.pem
pkey=/etc/letsencrypt/live/c.nllgg.nl/privkey.pem
no-stdout-log
simple-log
no-loopback-peers
no-multicast-peers
no-tlsv1
no-tlsv1_1

What can we do to try and debug the issues we experience?