Viber notification doesn't disappear
This happened to me in my VIBER FOR DESKTOP (Windows).
(In my case the problem was caused by an unread CALL on viber; but I couldn't find it!!!)
This is how I located and solved the problem:
1.  I clicked on the conversations tab.
2.  Then clicked on the small triangle that appears in the corner
3.  I chose from the options:
    "Unread messages on top"
4.  Voila!!! The conversation came at the top, I clicked on it and the 'unread message' disappeared!