No video calls on Viber on both S4 and S4 mini

Hi,

I have a GT-I9505 and GT-I9195 both of which on 5.1.1 (CyanogenMod 12.1).

I am unable to make Viber video calls on either phone.
Skype works fine.

When I press the video call icon, I see a black box where my video image is supposed to appear.
I can see the other person fine.

This is an issue on two different devices which makes me think it's not device specific.

Could someone please help?

Thanks.