Evangelos,
See JitsiVideobridge SIP Registration post and set the system properties manually as per this post.
This changes Jitsivideobridge from trying to register admin@IPofserver to extension-to-be-registered@@IPofserver
However as you can see from my post above, this also fails.
Can I ask how you log the registration attempt in your asterisk server - sorry don't know asterisk in much detail?
Clik here to view.
