Seems I need a little help by the Community.
latest Nextcloud 12, with Talk App > behind Reverse Proxy > behind Router with one Ethernet Interface. With 2nd Ethernet Interface it is within internal network. Access to internet is restricted through Squid Proxy. From the inside hosts accessing Nextcloud via internal interface, from the outside via public interface. When trying to use Talk and both users are outside, everything works. But it doesn’t work with internal users. My rough guess: NAT and Squid Proxy.
Any hint how to enable Talk for internal users? Own TURN Server?