Some more information as that might help diagnosing:
Nextcloud is installed in an apache webserver running mod_php, in a LXC on proxmox with a public IPv4 address. That server hosts many applications, like other websites, mailserver, jabber server, also coturn for TURN.
Next to that LXC is another LXC which hosts docker containers maintained through portainer. In that second LXC I run nextcloud/aio-talk which the nextcloud instance from the first LXC connects to.
The docker container is fronted by the webserver that’s also hosting nextcloud acting as a reverse proxy:
ProxyPass "/standalone-signaling/" "http://172.20.1.25:8081/" upgrade=websocket
#ProxyPassReverse "/standalone-signaling/" "https://172.20.1.25:8081/"
RequestHeader set X-Real-IP %{REMOTE_ADDR}s
RequestHeader set X-Forwarded-Proto expr=%{REQUEST_SCHEME}
Configuration in Admin → Talk is:
HPB-Backend URL: wss://cloud.anderdonau.de/standalone-signaling/ (green checkmark)
Shared secret double and triple checked.
TURN server is configured as well, tried tcp+udp (udp ports 3478 and 49xxx-65xxx and tcp 3478 allowlisted).
No recording and no SIP backend configured.