Jitsi using jvb with turn >2 participants

Hi there
I have the following config:
Server 1
External IPv4 and IPv6 DMZ - no NAT
Jitsi homepage 80,443 using nginx, jvb

Server 2
External IPv4 and IPv6 DMZ - no NAT
stun und turn server using coturn

Participant 1:
Behind AVM Firewall, guest net with filter

Participant 2:
Behind AVM Firewall, guest net with filter

Participant 3:
Behind NAT, no filter

Participant 4:
Behind NAT, no filter

I´ve red so many tutorials i cannot count it anymore.
Situation 1:
Participant 1 & 2 alone in a meeting - video works perfectly. Turn is used

Situation 2:
Participant 3 joins 1&2, no audio and video anymore

Situation 3:
Participant 4 joins 1,2 and 3
3 & 4 can see and hear each other.

In theory the jvb should route all traffic through the turn server and the same way back.
But there seems to be a problem with that.

I really hope you can help me with that.

No ideas here?

It seems, that my videobridge is not using the turncredentials

60275: session 001000000000000038: closed (2nd stage), user <> realm <stun.example.org> origin <>, local [IPv6ofTurnServer]:443, remote [IPv6OfClient]:60836, reason: allocation watchdog determined stale session state

Or am i totaly wrong here?