Appliance Access is Denied and Webex audio problems

Hi there,
Need your help and expertise.
We have an HA here with Sophos XGS2300 (SFOS 19.5.3 MR-3-Build652).
The following problem occurs in particular with Webex Audio / Video.

From external to internal and vice versa, the audio and video channel is established without any problems.
However, if the employees at the same location call each other directly, no audio/video channel is established.
Even if the corresponding firewall rule is set to Any / Any as a test.
No UDP is blocked by the DOS protection.


What we see is that almost at the same time there are "Appliance Access Denied" actions.
 

We do not have this behavior with MS Teams or any other VoIP provider.
We are currently at a loss as to what this could be.

Thanks for any help.
Stefan