Hi, i had the same problem with whatsapp and other apps on my M$ Lumia 640 LTE
Was on Windows 10 latest update for 2 days, uninstall apps, reinstall ... hard reset nothing worked..
I removed the SD card and reinstalled OS 8.1 Update 2 using Windows Device Recovery Tool
No customization on regional settings ... just signed in to live account and waited for the system apps to update
Installed Whatsapp and worked without any issues, didn't make restore of old backup, clean app