@tfoland temporarily log out of Yahoo on all browsers (and mail apps if applicable) on all devices and then try things again.
Then in Opera, in a normal window, you can goto the URL opera://settings/content/all, type yahoo.com in the search field and click "Delete displayed content". Then, you can goto the URL opera://settings/clearBrowserData and clear "cached images and files" for all time.
Then try things.
If you can log into your Yahoo account on some other device or browser, you can try going to https://login.yahoo.com/myaccount/activity/recent-devices/ and signing out all other devices.