Method 1 (Sure Fix but requires root):
Install a file manager like Root Manager
Browse to internal storage android/data/com.whatsapp
Delete the com.whatsapp… folder
Now install whatsapp from Google Play
Method 2: Removing Junk Files via Cleanmaster
Install cleanmaster
Uninstall Whatsapp
Now run Cleanmaster , search for Junk files and clean them. (Make sure to uncheck important files that are detected as Junk files by Cleanmaster)
Method 3:
When all fails backup all your apps and data and factory reset your device.