Tried using the Opera GX Sidebar My Flow to set it up, but repeatedly failed saying I needed to login to my account (already logged-in on both devices).

Fix: Opened a tab to opera.com/connect, scanned the QR on that page, done.

@afifhaziq I'm using Opera GX Desktop, but Opera Browser on my Android. Signed-in on both devices. I just know that going to opera.com/connect on my Desktop, then on my Android, navigating to the bottom-right Menu->Flow->Connect opened up a QR code scanner. I pointed that at my Desktop displaying the QR code (opera.com/connect) and it worked for me, albeit I was getting a different error code than you.

No idea why that's happening though, m8 ¯_(ツ)_/¯
I recommend you download the default Opera Browser on your phone, sign-in, then follow my steps above to connect to Flow. Just to see if it works. It could be a bug in the mobile Opera GX. I like the default Opera Browser over the Opera GX mobile, but you do you.

Best of luck, m8

Tried using the Opera GX Sidebar My Flow to set it up, but repeatedly failed saying I needed to login to my account (already logged-in on both devices).

Fix: Opened a tab to opera.com/connect, scanned the QR on that page, done.