xSP Storage Server Accounts that are members of the "Administrators" group cannot be utilized for backup. This Administrators group is the only group that has an internal flag set in the xSP Storage Server component that won't be allowed to be utilized for backup. When you go to create an account in xSP Storage Server's Configuration Manager application, in the Accounts tab, you will have the choice of assigning a new or existing account to the "Everyone", meant for use with the backup clients, or the "Administrators" group, only meant to manage the xSP Storage Server application itself. The Administrators group cannot be used by backup clients as an account to back up to from a client. You will get a generic cannot backup error if you attempt to utilize a xSP account that is a member of the Administrators group as assigned in the xSP Storage Server's Configuration Manager - Accounts tab area, specify that account in the xSP Device area of the backup client and then attempt a backup with that account.
Here is an example of attempting to utilize an xSP Storage Server account that is part of the "Administrators" group, in this case the account name is "administrator", as configured and shown in the Storage Server Configuration Manager application on the right, and the backup client that is attempting to utilize that account for configuration of an xSP Device, in this case the password that we are specifying is correct but the backup client cannot utilize an account in Administrators group so it keeps prompting for credentials. The dialog displayed contains "The credentials are incorrect or have been changed. Please enter new values to try connecting again.", as seen in this example:
During a Test connection or Configure xSP Device operation when attempting to use an account that is a member of the Administrators group (xSP Storage Server's own "Administrators" group), you may get this dialog error "The Backup Server does not have you configured as a valid user. Please contact your Backup Server administrator.", if that is the case:
No matter what we try we can't use an account that is a member of the Administrators group (xSP Storage Server's own Administrators group) in a backup client, it will fail to add or connect, Test Connection, and even worse, backups will fail if using an admin account. Here is another example using the user "Admin" who is part of the xSP Storage Server "Administrators" group, it keeps prompting for credentials when specifying the username and password even though we know the password is valid.
Additionally, if an xSP Storage Server account is changed some time after the xSP Device was already in configured and in place, from the Everyone group to the Administrators group, then the same scenario will occur, except that you may not notice anything other than backups will fail.
The resolution for this is to either change the account that you are attempting to utilize from the Administrators group to the "Everyone" group, by editing that user in the Accounts tab inside xSP Storage Server's Configuration Manager application, or create a new account there and assign that new account to the "Everyone" group and utilize that non-admin account in the backup client's xSP device configuration. You do have the ability to create additional groups and specify what abilities those groups have, such as quota settings, and licensed features to limit such as the ability to specify that the group cannot perform Copy, Disaster Recovery, or Virtual machine backup operations, etc., by using the "Group policy settings..." function in the xSP Storage Server Configuration Manager, to limit what group members can do and that is located in the "Group policy settings -> Licensing tab".