Why did I get the error "Password not valid" when accessing PayPal Developer?

PayPal returned an error on https://developer.paypal.com/ - Password not valid.

If you entered the correct password, the cause is likely a corrupted cookie.

To resolve this issue, clear your browser's cache and cookies.

Note: If you continue to have issues accessing https://developer.paypal.com/ after clearing your cache and cookies, you'll need to create a new account. PayPal currently can't recover developer accounts.