Whilst reading the Microsoft document Performance Tuning Guidelines for Windows Server 2008 R2 I read with interest this paragraph.
NTFS File System Setting
HKLM\System\CurrentControlSet\Control\FileSystem\isNtfsDisableLastAccessUpdate\ (REG_DWORD) 1 .
This system-global switch reduces disk I/O load and latencies by disabling the updating of the date and time stamp for the last file or directory access. This key is set to 1 by default.
Clean installations of Windows Server 2008 and Windows Server 2008 R2 set this key by default and you do not need to adjust it. Earlier versions of Windows operating systems did not set this key. If your server is running an earlier version of Windows or was upgraded to Windows Server 2008 or Windows Server 2008 R2, you should set this key to 1.
Disabling the updates is effective when you are using large data sets (or many hosts) that contain thousands of directories.
Caution: Some applications such as incremental backup utilities rely on this update information and do not function correctly without it.
I am not sure if the answer is set the key to 0 (zero) or if you need to update your backup software to one that is aware of this change to Windows and NTFS – but this could be a reason as to why your backups are failing.