Nextcloud Talk: black screen most times, screen sharing somehow possible

Hello together,

I started using nextcloud some months ago and I am totally satisfied with the functions I used up to now. Last week I’d like to start with nextcloud talk. So I added the actual version of the app and tried to set up a talk within my local lan (behind a firewall/router using a local lan 192.168.10.x).
I could start a talk and join from two users, git text messages via talk, saw my own video - but not the video of the counterpart and couldn’t talk via the connection.

So I looked around and set up a coTURN server on a raspberry pi linux system within my local network. It’s reachable from my nextcloud server, that’s running on a hosted webserver. Checking the connection the system says, it’s ok. I also changed the SIP server to the coTURN server - on port 3478 also, like the coTURN - and its reachable via a port release on the main router.

Afterwards it’s possible to share a screen and the counterpart can see the screen wit some minutes delay. So - let’s say I have a very poor internet connection, 2,9 MBit download, 2,1 MBit upload. But the connection should be set up directly between two browsers, as I assume - and they’re both within the same LAN / WLAN. But however - the video and audio signal will not be shared.

There are some suggestions to solve the riddle and set up a successful talk between 2 or 3 participants?

Thank you in advance!

[details=“Support intro”]

Nextcloud version: 18.0.3
Operating system and version: Ubuntu 16.04.6 LTS x86_64

The output of your Nextcloud log in Admin > Logging:
…/tmp/ was not available. was corrected.

The output of your config.php file in `/path/to/nextcloud` (make sure you remove any identifiable information!):

$CONFIG = array (
‘instanceid’ => ‘yyyy’,
‘passwordsalt’ => ‘yyyy’,
‘secret’ => ‘yyyy’,
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array (
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