[16.04] Skype doesn't coect properly

Hi all.

I'm having a problem with Skype 4.3 on Ubuntu 16.04.
It seems it caot coect properly as I don't see my contacts online (even if they are) and they don't see me either.

The weird thing is that sometimes, when we are both online in front of the chat window, we can chat normally!
But most of the times it doesn't work actually...

I don't think it's a firewall problem as I never had any issue on Ubuntu 14.04 (same firewall rules) and I still have no issue on my other clients (Ubuntu 14.04 this time).
Maybe it's not even an Ubuntu issue, most probably it's Skype server side... but I'd just like to know if someone else has the same problem and, possibly, if we can fix this somehow.

Thank all.