EC2 Ubuntu 14 default password

I have an EC2 instace running Ubuntu 14 which I regularly connect to using SSH.

Now I'm trying to connect to this instance using Remote Desktop from Windows, as shown here.

In the Login to xrdp form, I typed "ubuntu" as the username, but I don't know what the password is. I've never messed with the password and never needed it. I have tried both "ubuntu" and "" (blank) but got the same result:

xrdp_mm_process_login_response:login failed

Does anyone know what the default password of Ubuntu on EC2 is?

If it is either "Ubuntu" or "" then what causes the problem?