I have a Nokia Lumia 1020. It just stopped syncing my Yahoo email to my Yahoo app on the phone. It keeps asking for my password and telling me it's incorrect.
- On your phone go to Settings → Email and accounts. Find Yahoo email, press select option to delete.
- On a PC open a browser and go to Yahoo email. Click on your account name and select "Account settings". Go to "Security of account" and scroll down to where says "Allow apps with less security". Enable this and log out from your email.
- On your phone again go to Settings → Email and accounts. Add your Yahoo account, enter your email and password and sync to finish.
IMPORTANT: when on PC or on the phone or app you get a message about device with less security, select "I understand the danger".