The version of the iOS installed on your phone is the latest, which means you could face a problem related to compatibility of the application with the operating system. I recommend you leave everything as it is when it comes to file management and re-install Viber or check for updates. There could be an option to apply a new update in order to fix bugs and errors.
According to various sources, this was a known issue resolved in an updated version.
Use the store application to install the latest version of Viber.