I also opened a Bug Report (ANDEX-78330) about this.
When toggling Sync and then entering the Passphrase it says "Temporary server error, Please try again later."
This is a new issue seemingly introduced with the latest version of Opera.
I get it on one device (where I did yesterday a fresh install) so I checked another device (which was still on the previous version, 56) and it didn't had any issue, updated from the Play Store and it has the same issue now!