Since everybody is giving complicated or long fixes i figured i'd just say the quick (30 seconds)way to do it.
All you need to do is have the update that came out within the last 24 hours. Then go to your whatsapp ANDROID APP SETTINGS and then press "clear cache".
This way does NOTHING to your Whatsapp, no need to reinstall it, no need to resore your message, nothing. You tap the button and it will happen instantly, then go back to WhatsApp and the web option will be in the menu crop down button thing.
Takes 30 seconds total.
Tried and tested on 3 phones.
edit: Make sure the app is closed first, using the force close button or swiping away.