"You've left the call" when joining external web conferences (e.g. SfB online)

Hi,
A customer has a problem if he participates in external meetings (e.g. SfB Online) via the SfB Client. If he activates audio/video, he will leave the meeting after a short time ("You have left the meeting"). The employees can be in the internal network or in the Internet, the result is identical.

Environment:
Skype for Business 2015 environment with 1x FE Standard, 1x Edge, 1x WAC, 1x Reverse Proxy (NetScaler)

What did I check?
- Firewall rules look fine (https://docs.microsoft.com/de-de/skypeforbusiness/plan-your-deployment/edge-server-deployments/edge-environmental-requirements)
- With OCSLogging I didn't see anything really helpful with the Case AudioVideoConference (is it the right one for that case?)
- Different constellations
- Wireshark and Fiddler - I could not recognize anything

Any unusual features during the tests:
- 504 Timeout message

- 504 Service unavaible message

- In some constellations it works
o It always worked in the test via a WAN set up via my mobile phone
o Internally it worked from time to time when I clicked "retry/rejoin"

Do you have any idea what could be wrong with that? When I think of the error scheme, I'm somehow thinking of a firewall topic, but I can't specify it more precisely.

How can I narrow it down? Is there a better ocslogging scenario or other possibilities (e.g. on the client)?

Thanks in advance

Patrick