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.
@treytango There is no such option in Opera, each installation will generate a folder with the device name but you can try this.
In Bookmarks manager > other speed dials you can remove the folder related to your no longer used devices.
Despite all seems to be new, try renaming your profile's folder.
OMenu > About Opera
Follow the path to the profile with your folder's explorer and test it.
Windows 10 (x64) | Anniversary Update
Opera Stable · Beta · Developer
Opera Test profile | Opera Reset / Back up Linux · Mac · Win