WebRTC public IP instead of local IP

Hello dear Asterisk implementers

I have been stuck for like 10 hours (I know it’s not much but still, I need to get going).
First some information: I am developing a WebRTC VoIP application with Sip.js and Asterisk. Asterisk and my webapplication are running in my LAN, so no NATting (for now). Right now, I can call to a SIP Phone, it is ringing, but I can’t answer the phone. I know I should ask the Sip.js guys for problems, but the only question I am asking is: how does it come that my contact/via-address is my public IP?

This shouldn’t be possible, right? I know it should be (one of) my local IP’s. If I answer on my softphone, ICE connection is OK. I can hangup (sometimes I have to wait a few seconds).
If you need anything, just ask. It’s just so strange.

Zoiper has placed that in its SIP REGISTER.