Perhaps a bug? Logged out at xbox.com
I may be doing something wrong but anyways here's what I did...
I went to:
I also logged in, within a few seconds it booted me out.. Not sure why I never had any problem on any browser before, I first tried it with the official release of Vivaldi now I'm on the beta to see if it made a difference and the answer is nope.
Any suggestions would be greatly appreciated, or if this has been reported before let me know since I only started to use the browser yesterday but I gotta say it's the quickest one I've ever used.
You could try to spoof user agent too see if it works setting edge or chrome in the extension.
Maybe it's not the issue, but sometimes it works.
ONLY if you ARE NOT using vivaldi as main browser you can also try to remove all stored data (cookie, cache, saved password ecc ecc) in tools menu (vivaldi button) and see if it helps.
Thanks for your quick reply @Hadden89
I discovered what it was, I had "block 3rd party cookies" turned on... once I turned it off, everything is fine now.