Still no solution here. Android isn't a problem, but my iPhone doesn't connect yet.

@korol78 Why do you not click CTRL-N?

On Android, Opera works better for me. On my iPhone Opera also seems to be better, but it's a close race. On desktop they are on par, with Vivaldi being slightly in front. But that's very subjective.

Flow still doesn't connect my iPhone, unfortunately.

@leocg I did it both ways. No difference. As I said: Going to messenger.com triggered the same issue, also in another browser. Really seems to be caused by Facebook.

@leocg I've tried to do this: I'm already logged in into Facebook on Vivaldi. Now, I headed on to www.messenger.com - the URL which also Opera in the sidebar will probably load for the Facebook messenger. And, to my big surprise, this also failed, too. Not only had I to give in my credentials (although being logged into Facebook already), it also showed me: "This page is empty."

I guess this may happen because of Meta's attempt to combine Facebook and Instagram messaging into one application.

@leocg Even with a clean install without my personal settings it doesn't connect. I'm shown a blank page with the word "Fehler" (German for Error).
So it's either something on Opera's side or maybe even more plausible on Facebook's side.
I've experienced something weird with Facebook in any other browser as well: When I try to switch from my personal account to a professional one, that's connected to my private one, it takes quite some bit to load, as if there were nothing happening at all. This appears on Vivaldi, Edge, Firefox and Safari alike.
To me it seems that Facebook has changed something internally about connecting different profiles and maybe even sites.
And maybe, the sidebar issue could be something that the Opera team may investigate.

@leocg Opera One, latest version. It only concerns the sidebar. The regular page in the window isn’t affected.
When I give in my credentials and allow for logging in my Facebook app, I get these results in the sidebar. Still I blame the third party cookies thing somehow, maybe I should enable them before the the very first log in.

I‘m facing issues lately when trying to log into Facebook Messenger in the sidebar after a new install. I‘ve enabled „block third party cookies“, but even allowing them makes no difference.. I get a blank page or at best a „normal“ Facebook window. Logging into Facebook in their regular web page doesn’t help either. Refreshing the sidebar page mostly logs me out, too.
I‘ve done this so often before, but as of now it doesn’t work.
Any Facebook related problem or coming from Opera? I don’t know.

It looks like a flaw in the iOS app, because it didn't work with another laptop and a different OS as well. Hopefully they fix it with the next update.