Hi, I also had exactly the same experience yesterday, in identical circumstances to those outlined by David Cowling and freekidolpinz.
I also emailed Viber and they initially said nothing could be done. I persisted and have received the following response, which I'm yet to try:
"We have been working hard to find a solution for our users; we are now testing a possible solution that may allow you to retrieve some of the messages on your account if you have performed an iTunes back up before Viber updated to version 5.4.1.
Unfortunately, this procedure may not work if you backed up your phone via iCloud instead of iTunes, or if you did not back up your device at all.
If you backed up your phone via iTunes, please try the following:
- Disable automatic updates on your device before attempting to restore your iTunes back up (so Viber does not update back to Viber 5.4.1)
- Make sure you have backed up your data via iTunes before the 28th June (i.e before Viber 5.4.1 was released).
- Restore your phone from the back up. If you’re not sure how to do that, please see this link: https://support.apple.com/en-il/HT204184
Please let us know if restoring Viber to version 5.4 (or earlier) via iTunes restored your messages to your account.
If so, please do not update to Viber 5.4.1 for the time being.
If you did not create a recent back up of iTunes, I’m afraid there is currently no other way to restore these messages.
Best regards,
Amie B.
Supervisor
Viber Support Team
Quick question?"