So I just tried to test the sync feature on a number of devices.
I have tried Both my Wi-Fi Access points and my cellular network on the G2.
The error message I receive when I try to log in or create an account on an android device is the one previously mentioned: "Temporary server error. Please try again later."
I'm in Sweden.

• WORKS Windows 7
• DOESN'T WORK LG G2 d802 (Android 5.0.2 stock rooted) (Opera 42.7.2246.114996)
• DOESN'T WORK Honor 8 FRD-L09 (Android 7.0, non-rooted) (Opera 42.7.2246.114996)
• DOESN'T WORK Galaxy Tab GT-P1000 (Android 4.4.4, CyanogenMod 11 rooted) (Opera 42.7.2246.114996)
• DOESN'T WORK Galaxy Tab 10.1 GT-P7500 (Android 4.0.4 stock rooted) (Opera 29.0.1809.93516)
• DOESN'T WORK Galaxy Tab A SM-T585 (Android 7.0 stock non-rooted) (Opera 42.7.2246.114996)
• DOESN'T WORK Galaxy Tab S SM-T800 (Android 7.1.2, LineageOS 14.1 rooted) (Opera 42.7.2246.114996)
  Tested on 28-08-2017
  I could possibly test on two more android devices.

I spoke too soon. It isn't actually syncing. Sorry!

Actually it is syncing. You just cant be using the two devices at the same time. Only one of them has working syncing when doing it this way. I'm guessing I copied over some identifier that needs to be unique. But At least I figured out it's possible to sign in and that it's not a "server error".

My guess is that the opera app generates a identifier for the device and sends it to the server when signing in through the app. On newer android versions that identifier doesn't look like the server expects it to and the server just throws back a generic error message.

Of-course if you log one device out you log them both out.

I spoke too soon. It isn't actually syncing. Sorry!

I found a workaround. Find an old device that has opera installed and you are still signed in to to. Make a backup of the app and its data with an app like Titanium backup. And then go to your newer device and restore the app and its data there. Now you have an opera install that is signed in to your opera account. Good luck!

I seam to have better luck with Opera mini(28.0.2254.119213).
The sync feature seams to work on Opera mini with the LG G2 and Galaxy Tab S at least.
Minis sync didn't work with the Galaxy Tab. I have not tried the other devices yet.

So I just tried to test the sync feature on a number of devices.
I have tried Both my Wi-Fi Access points and my cellular network on the G2.
The error message I receive when I try to log in or create an account on an android device is the one previously mentioned: "Temporary server error. Please try again later."
I'm in Sweden.

• WORKS Windows 7
• DOESN'T WORK LG G2 d802 (Android 5.0.2 stock rooted) (Opera 42.7.2246.114996)
• DOESN'T WORK Honor 8 FRD-L09 (Android 7.0, non-rooted) (Opera 42.7.2246.114996)
• DOESN'T WORK Galaxy Tab GT-P1000 (Android 4.4.4, CyanogenMod 11 rooted) (Opera 42.7.2246.114996)
• DOESN'T WORK Galaxy Tab 10.1 GT-P7500 (Android 4.0.4 stock rooted) (Opera 29.0.1809.93516)
• DOESN'T WORK Galaxy Tab A SM-T585 (Android 7.0 stock non-rooted) (Opera 42.7.2246.114996)
• DOESN'T WORK Galaxy Tab S SM-T800 (Android 7.1.2, LineageOS 14.1 rooted) (Opera 42.7.2246.114996)
  Tested on 28-08-2017
  I could possibly test on two more android devices.