Plugins & SpeedDial Thumbs
Hey, I just noticed that the SpeedDial preview thumbnails seem to ignore installed plugins when loading. I know that plugins are not officially supported yet, but hey In my case I have installed the ghostery plugin, and using vivaldi:extensions it is listed as enabled and configurable. And it works, because its Ad-Blocking is working. But when I added www.spiegel.de to the news folder on my speeddial the large Ad-Banner on top was quite visible in the thumbnail, as opposed to being blocked on the actual page when opened.
I think (but am not sure) at least some of those thumbnails are generated differently than what is displayed on your screen in the browser window (well, unless I'm overlooking something, it seems like that must be the case for the one you describe). And I don't know enough about Ghostery's ad-blocking to know if it supposedly blocks ads from getting on the machine or just from displaying. (If it is the latter, that might explain the banner ad displaying in the Speed Dial thumbnail.)
You might be able to tell a little more about it by looking at the TopSites Thumbnails found at vivaldi://thumbnails. (These are stored in the Top Sites file in your profile Default folder.) Notice there are Forced URLs and Non-forced URLs with Thumbnails, as well as Forced URLs and Non-forced URLs without Thumbnails. I'm not sure what that means exactly, but you might have more of an idea after looking at them.
This is or was a problem in ChrOpera too. I believe it's because the thumbnail is created by an internal page where extensions do not have access. There may be a workaround.
Ghostery blocks them.