I think, I solved this. I had same issue.
You have several mycloud_USERNAMEs. Say they are mycloud_USERNAME1, mycloud_USERNAME2.
When you log in from web ( https://www.wd2go.com/) you log using email ID. Each one will have equivalent user ID.
On My cloud dashboard, on tab ‘Users’ or ‘Cloud Access’, you will see all user IDs. One of them is having admin rights and it is mostly first in the list marked with small green check mark. Suppose it is mycloud_USERNAME1 and it’s password you set is password1.
When you want yo access as you mentioned put user ID mycloud_USERNAME1 and leave password as blank. Don’t put password as password1. It is only for accessing from web.
The manual on page 56/126 have below info.
As the WD My Cloud device owner (admin), you have a special user account (admin) that gives you administrative privileges. These privileges enable you to set up and configure the device to your specific needs, as well as give other people access to your personal cloud. You have the power to determine exactly what other users can access. As a default, the admin user name does not have a password. You can add one at any time."
This finally worked for me after giving several trials like you.