Backup Validation issue on Vista

I have two Windows Vista SP2 machines, both running True Image Home 2011 Update 1 (build 6696), English UK.

On both machines I use Acronis to backup various EXT3 and NTFS partitions to a NTFS share exported via Samba from a Linux Fedora12 server. The partitions are similar on both machines in size, contents and type. I use sector-by-sector, including unallocated space, for all my backups. The two PCs are connected to the Linux Samba server through the same router in the same network.

On Machine1, Acronis works fine: backups of 30GB to 200GB are created and validated in a resonable amount time: both operations take about 10 to 25 minutes each for all partitions.

On Machine2, Acronis creates the backups just as on Machine1, but then the validation takes absolutely forever. The time estimates are totally absurd (it can show 4 minutes or 5 days for the same backup) and more importantly the validation never ends, also preventing other backup in the queue from running. I always end up cancelling the validation, after waiting 4, 8, 12 hours.

Machine2 also worked just fine (validation included) with the same version of Acronis until about 1 month ago, when I upgraded it too to SP2. Machine1 never had this problem, pre- and post- SP2.
I solved a similar slowness problem (but also involving the backup operation) about 1 year ago on a XP SP2 machine, by tweaking some TCP buffering parameters in the registry; I believe this to be an issue with Vista rather than with Acronis, but right now I wouldn't know where/what to change to fix it, as I'm not an expert with Windows Vista internals.
The issue happens with all backups (the Vista partition itself or others on other disks) on Machine2, and both when running Acronis on this machine only or also concurrently on Machine1.
The Samba server on the Linux box works just fine; I'm able to do operations from both machines on the Backup share and other shares at all times including when Acronis on Machine2 is not working; by tracing the system calls I can see the samba server process is indeed continuously reading from two file descriptors (ie. it's not stuck or hung) when its only client is the Machine2 "validating" a backup.

Any hint or help would be greatly appreciated.