Can't find a similar and weird issue reported in the Vivaldi Bug Tracker yet.
Can you please test with a clean (no extension, no settings change) new profile and if the crash is reproducible, feel free to report it and join a crash log.
@ramon Seems to be resolved, for now. The only hint I got from the error log of Minecraft was some memory shortage, even though I had enough RAM. So I checked the page file and upped it up. It seems to have helped. MC is running, and I'm typing this in Vivaldi.