I deleted all instances of ‘amarok’ in
$HOME/.kde4/share, and dropped the ‘amarokdb’ database, and dropped ‘amarokuser’, and re-created everything, but it still wouldn’t connect.
It turns out the solution is simple. Just change the password in the Amarok settings page from ‘password’ to something else (e.g. ‘password123’). Change the ‘amarokuser’ password, too, in mysql. Restart Amarok, and they can connect.
This is a bug in Amarok, as it doesn’t have a default password configured, one must be explicitly set. Source