Same issue,the synchronization doesn't work anymore.
I have installed a new computer with Windows 10 and Opera, impossible to sign in. It looks like the master password is not used anymore if you have set up previously.
I really like Opera, but the Sync feature is the weakest part of this browser.
@spaceshipone Try clearing the cache and cookies of the browser after logging out of your opera account if you can. 9 out of 10 times that will fix most browser problems.