Try rebooting your device, maybe it will help.

Sorry, this is the first time I hear about the issue like this. Is this happening only in Viber, or in all other apps (Chrome/Web browser, SMS messages etc.)?

If yes, then your keyboard app is the cause, so depending on which one you use, you must go to Settings >> Apps >> ALL, and locate your keyboard. If you use some 3rd party keyboard app, uninstall it and revert to factory version. If it’s already a factory keyboard app that comes with your phone, you can try to revert it to factory version (uninstall all updates etc.), but you will also lose all the settings, so you’ll need to reconfigure it again, like input languages, international settings, dictionaries, auto-spelling and so on. Also, you might try installing some 3rd party keyboard app from PlayStore.

If it happens only in Viber, and you alredy re-installed Viber app, then I simply don’t have a clue what’s happening. You might wish to report the issue to Viber devs directly and wait for their response.

I can’t seem to find any solution in its settings.