Conference is broken when internal users join - only VPN for external helps

I did setup a meet server in our DMZ.
When external clients do connect - fine.
When internal clients do connect - fine
But when just a single internal client joins a meeting, audio and video is broken.

The only way to have a meeting with internal and external clients is to force every external to use our VPN.

My guess is that there must be a special setting of ip routes at the jitsi server. But which one?

Setup is as follows:
internal clients ==internal firewall== DMZ with jitsi meet ==internal VPN endpoint on external firewall== www with external clients.

Thank you in advance - Chrizz