Problem with Automated Backup of NAS Folders

For a few weeks now, I've been having a strange problem with an automated backup routine, which backs up a group of folders which are located on my NAS drive. Each time for the past few weeks, the backup routine fails with the following error messages in the Macrium Reflect log file:

Error: Folder '\\DS413\RMHShare\My AudioBooks' could not be found
Error: Folder '\\DS413\RMHShare\My Charts' could not be found
Error: Folder '\\DS413\RMHShare\My Music' could not be found
Error: Folder '\\DS413\RMHShare\My Music - Christmas' could not be found
Error: Folder '\\DS413\RMHShare\My Music - Patriotic' could not be found
Error: Folder '\\DS413\RMHShare\My Music - RVCC' could not be found
Error: Folder '\\DS413\RMHShare\My Music - Videos' could not be found
Backup aborted! - Folders not found.

However, if I go right into Macrium Reflect and manually launch that very same backup routine, it finds those folders without any problem, and the backup routing will run perfectly each and every time.

This backup routine has been in-place for a *long* time now, and just recently started having this recurring problem. Any ideas what might have suddenly started causing these problems?