Looks like upstream issue, not a OMV.
Vivaldi is a proprietary browser and to watch netflix you need proprietary Widevine plugin. Widevine in Linux world is provided only on Google Chrome and Firefox (opt-in). Rest of browser dont have license for this (money, legal reason, etc).
In most of Linux web browsers you need manually copy this plugin from chrome to dir in your browser.
Vivaldi with version 1360.4 implement scriopt to auto download this plugin. More info here: https://vivaldi.com/pl/blog/snapshots/better-widevine-eme-support-for-linux-vivaldi-browser-snapshot-1360-4/
also make sure you have working h264. If h264 not working in your browser then widevine wont work too,
So deal with h264 and then try again with widevine.