Updated for OnApp 5.2 (7th December 2016)
The error is shown if there is a mismatch in credentials between vCloud Director and OnApp. This can be resolved by updating your User and editing the vCloud Credentials associated with your account.
This can be achieved by going to your User > My Profile and scrolling down to the "vCloud Credentials" section on the left hand side. Here you can update the vCloud Director password associated with your user.
The most common cause of this is if the user account inside of vCloud Director has had the password changed without it being updated in OnApp.