Comment 7 for bug 1047588

Hi all, and sorry for the mess!
  This bug is caused by a change in the online-accounts-signon-ui, which is now more strict about the URLs it handles; in particular, all non-https URLs (such as "http://www.mardy.it/oauth2callback") are opened in the default browser.
At the same time, we updated the account plugins (source package is online-accounts-account-plugins) not to use that URL; however, all Online Accounts which you might have already created are not automatically updated to use the new settings.

The bug for tracking this is bug 1047191.

*SHORT TERM SOLUTION*

Delete all your accounts from Online Accounts (which is found in the System Settings), and re-create them. Don't worry, this won't cause any data loss.
Sorry again for the inconvenience!