TI 2009 Build 9809 - Unable to create volume snapshot.

I'm getting the fllowing error when I run an incremental or differential backup on my system.
OS: Win XP SP3 with all latest updates.
True Image Home 2009 Build 9809

event code="502" id="8" level="4" message="Operation with partition 1-0 was terminated
Details:Unable to create volume snapshot (0x70021);Tag = 0x2CBDD167CBCA958
Failed to enumerate open files on volume. (0x10C451);Tag = 0x14181C22EF45ADD0
Invalid access to memory location (0xFFF0;code = 800703E6;Tag = 0xBD28FDBD64EDB8B9"

snapapi.dll is version 3.7 build 462

The backup destination is a Thecus N5200Pro NAS and is reachable.

FULL backup runs fine.

How to I resolve this please?

Thanks