@janrif One of your malicious/misbehaving extensions is doing this.
You need to disable all of them.
Them enable one by one followed by a browser restart and waiting until it triggers.
Or using some logic, you might be able to discern what extension might be causing it - what's the most recent/most shady extension you've installed?
Something relating to crypto perhaps? Or investments?
Did you actually try what was suggested above - launching Vivaldi with --disable-extensions ?
what's the practical application for Vivaldi's "Ad/Tracker Blocking"? I'm still not sure what it's supposed to be doing.
It's supposed to block ads and trackers and does a reasonably good job of it. It is not meant to and cannot block malicious extensions you've installed from opening new tabs when it feels like it.