Hi,
This is a member of the Viber R&D Team!
Thank you for your feedback on our application and service. We are glad to hear about users' experiences.
Viber is unfortunately not fully compatible with MIUI at this point... :/ Did you try to uninstall>reinstall Viber?
Please feel free to let us know your thoughts, questions and comments, we'd be happy to assist.
Thanks,
the Viber Team