Hie.
This may be a bug that is present in the Viber application.
Independent of which device you are using, some applications may run incorrectly on a range of multiple devices and you may find that even some one with exactly the same mobile as yours may not be having the same problems.
This can usually be fixed with a simple update or re-installation of the application. If it should persist further after doing this, there may be another underlying cause such as a specific sequence of buttons you keep pressing and this causes the app to misbehave (which is fairly common), or some software/certificate conflicts.
