As I suspected.
The updated version is currently in stable and testing; it’s just a question of waiting some time for it to reach Stable.
Otherwise, there are workarounds that can be employed, as mentioned in this thread:
1. You can also simply wait until it made it’s way to stable
2. You can also do like this (the more manual approach).
3. I switched over to Testing for now & Viber is working perfectly.
4. Plus, one or two more.
Regards.