If you select the server, right click, choose "Details". On the details page you will see the listing of the open shares that CIFS is reporting. Each of these shares has a credential associated with it, and the default is to use the users windows logon account.
That default needs to be changed to use the credential used during the setup of the job. Once I set the proper credential on each share, the backup job completed. The synology connection logs show the correct user being used.
FYI. This is using an IBM TS4300 with an LTO9 tape drive. This configuration required the use of AVAGO SAS3 3008 StorPort card. We had an ATTO card but it could not speak to the IBM device and could not manage the control path.
