Skype video call drops after 10 seconds

[ohne]

Hey!

Since Skype is quite popular, I can't imagine that this is a common problem so it seems that I have a configuration problem...

Problem:
User 1: Skype on Amazon Echo Show and consumer router/access point
User 2: Skype on iOS/macOS and opnSense (Proxmox KVM) + piHole (LXC) + Ubiquiti AP

Skype video calls can connect just fine, but after 10 seconds the picture freezes and 10-15 seconds skype drops the connection. The error can be reproduced on mac and iOS official skype apps. When User 2 changes the connection to mobile data on the iPhone, calls are working fine and there is no freeze.

Setup:
Opnsense is virtualized on promox.
There is one WAN and one LAN port, WAN is connected to a cable modem which is in bridge mode.
Opnsense gets a public IPv4.

Rules:
Private + bogon blocked on WAN (disabled both - no success)
GeoIP blocked countries on WAN (were disabled - no success)
SPAMhouse Drop + eDrop on WAN (were disabled - no success)
Refelections for NAT: on
Reflections 1:1: off (tried to enable - no success)
Automatic outbound NAT reflection: on
All Skype domains are whitelisted on pihole. (Also Pihole got disabled - no success)

Log:
During the first 10 seconds of the Skype call there are a lot of incoming connections from the IP of User1 which are blocked by the general deny rule.

I installed the UPNP plugin, but it doesn't help ether.
Is there something else I can look into? Video calls via Amazon Alexa App from User 2 to User 1 are working fine. It's only Skype...