Sorry for the trouble, the team is looking into this issue. In the meantime please try this workaround that we share here: https://help.vivaldi.com/article/user-agent-spoofing/
It won't work on the web panel unfortunately. Thank you for being patient with us. Also this topic is being discussed in so many topics that I'm closing this one and we'll update you once the fix is out here: https://forum.vivaldi.net/topic/37195/vivaldi-2-5-breaks-whatsapp-web