Debian – Viber on Debian 9 – No internet connection

I've got a brand new HP Probook 430. Debian 8.3 stable was not able to meet requirements of new hardware. So I've upgraded to Debian testing (Stretch) and put a new kernel as well – 4.3.
https://wiki.debian.org/DebianTesting + https://scottlinux.com/2013/11/23/how-to-install-newer-kernel-from-debian-wheezy-backports/

I've installed a proper driver for wifi – iwlwifi. I've a connection with a cable as well. Both cases – 50 Mbps nice Internet.

I've downloaded and installed successfully Viber 64 bit. I can start it and type my mobile number. At the next screen I've a notice: "An Internet connection is required to activate your Viber account. Check your connection and try again".

I've a PC with Debian 8.3 running Viber successfully. Both computers are connected to my router. PC – with a cable, laptop – with wifi n.

I've same Viber problem when I connect my laptop with a cable.

Any suggestions?

Best,
Simeon