"Multiple connections to a server or shared resource by the same user, using more than one user name, are not allowed. Disconnect all previous connections to the server or shared resource and try again"
This issue is caused by a confusion/collision in network mapping by Windows.
More info : https://support.microsoft.com/en-us/help/938120/error-message-when-you-use-user-credentials-to-connect-to-a-network-share-from-a-windows-based-computer-the-network-folder-specified-is-currently-mapped-using-a-different-user-name-and-password
1 . Use IP Address instead of UNC path when adding Network Location to Backups-
- a. To obtain the IP Address of Network Location : Open Command Prompt as Admin - Ping "Folder or Device name"
- b. Use the IP Address as the Network Address in the Backup client
OR you can try
2. Removing Mapped connections in Windows Explorer - Add Connection to Backup Configuration - then Remap to the local machine via Windows Explorer.