This thing just happen to me like 5minutes ago, suddenly my WhatsApp web is logged off and i need to do a device link by using a QR code generated by WhatsApp web on my mobile phone.
But heres the problem, my phone have a very bad rear camera. It’s unable to stay focus on the QR code thus making me unable to login.
Workaround is quite simple, i just need to zoomout on my browser (press ctrl and “-” sign) and put my mobile phone camera close to the monitor until finally my WhatsApp mobile able to read the QR code perfectly.
Weird, but somehow it works lol.