I changed the UA on Frirefox, then whatsapp web started working. Firefix even showed desktop notifications.

BTW, you cannot have two web sessions going on at the same time. Once you start a new session on one browser, other detects and prompts

"WhatsApp Web is open on another computer or browser. Click “Use Here” to use WhatsApp Web in this window."

also, it needs phone to be connected to whatsapp service. So it sync from phone to your browser.