Comment 3 for bug 1313466

This was being caused by a Yahoo account on my machine. Removing the account and re-adding it fixed the problem.

The following fix worked for me:

Symptoms of a faulty Yahoo account in Online Accounts manifest as the following: When you go to System Settings > Online Accounts, click on your Yahoo Account in left column, then click on "Edit Options", there will be NO information displayed - window will be blank instead of the expected username & password fields.

To fix a faulty Yahoo account in Online Accounts you need to delete the account and re-add it: Go to System Settings > Online Accounts, click your Yahoo Account in left column, click "Remove Account", then click "+ Add account..." The username & password fields will display as expected and you can fill out your account info. Congratulations, the "Enter your credentials" popup is now gone when you reboot.