I go to wunderground(dot)com and get a white screen.
Hi, work fine here with latest snapshot.
And it's working fine here with the latest 1.13.1008.34 Stable version (64 bits). I have observed that in recent weeks, some of their local radar displays have sometimes been doing peculiar things... including the odd white-screen or two. Clearing Vivaldi's cache cleared that up once for me, and another time, it cleared up itself after I waited an hour or so to revisit the site. I had written off these peculiarities to Wunderground messing around with their site code lately, since within that same timeframe, I had to edit my custom-configured bookmark after they altered how they parse the settings data at the trailing portion of URLs for local radars; likewise, it's happened across the last 3 stable versions/updates. Of course, I'm not sure if you're seeing some of the same things, but you could try clearing Vivaldi's caches.
Without any problems, Vivaldi last stable, win 7 32