It's a long read at https://www.viber.com/terms/ but it appears that loss or deletion is not a violation of the agreements.
Even here, moderators can remove messages that are spam, border on spam and for other reasons. Technical issues like failing servers, bugs in the code can result in message loss.
-> In other words, you don't have a leg to stand on. Try another messaging service?
Hello community. I would like to report Viber about a deletion of my messages.
I' ve allready messaged them, but there is no solution. Do you know at which authority could I file a report?

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