i'm having the same issue as yourself, I have a new computer and my last one had windows 7, now with my windows 10, Opera is Signing in but stays with nothing and the Yellow/orange exclamation mark stays on the account button top right.
what is weird is that I have another windows 10 computer that has been installed a bit earlier and have opera and the sync work on it.
so to list them up:
Windows 7 with opera syncing
Windows 10 that was installed many months ago with opera Syncing
Fresh Windows 10 that I just installed can't sync.
what is up with that