I saw some threads lately about synchronization and it seems it's broken for a lot of people.
I had reinstalled OS today and I've ran into this problem as well. Opera just won't synchronize no matter how many times I try. It just keep showing these things:
Under menu: https://i.imgur.com/6AOE6M0.png
opera://sync-internals shows this: https://i.imgur.com/bU1WJFx.png
https://sync.opera.com/web/ shows this: https://i.imgur.com/e3unrbK.png (please note, that it shows the info sometimes and sometimes shows nothing, e.g. this was shown when I just installed opera today - https://i.imgur.com/JGCnP8v.jpg )
I'm unsure if it's operas servers that's broken (because it shows SERVER_RETURN_TRANSIENT_ERROR as error) or what, but it's really getting frustrating.
I've tried older opera version (just in hopes it might fix it) but to no avail.
I've also tried making new account and see if synchronization works - it does not.
Has anyone found a fix to it?