Hi Guys,
I fixed Viber on my Lumia 920 today.
I had push notification issues for quite a few months now and I was able to find a way around this problem.
Here is what I did:
1. In Settings / Background tasks, I only allowed two applications to run in the background.
2. I uninstalled Viber from my phone.
3. In Settings / Storage check, I cleared the temporary files.
4. In Settings / Ringtones+Sounds, I unticked/unchecked the "all other notifications" (I saw that Viber will use its own notification sounds for push notifications).
4. I performed a soft reboot (holding down the volume down + power button).
5. After the reboot, I again cleared the temporary files in Settings / Storage Check.
6. I re-installed viber.
I am not really sure how it worked
I hope this info helps.