Flash failure on some sites
Some sites that stream live Flash video fail to open unless the option "Always Allow Flash" is enabled. The default is "Detect and Allow Important Flash Content Only". I had removed Vivaldi twice because of this problem with favorite sites, as a new user. Then on third install found the switch in settings. I suggest leaving Flash option "Always Run" on by default, until the day Flash is end of life.
At least yours seems to work now. I enabled all Flash content but still get "couldn't load plug-in" on everything now. I have to use Chrome again (yuck) to view anything with Flash.