I have been a user of Acronis for many years and was very pleased when they were amongst the first to ensure that a Vista version was released in time for the actual Vista release.
I am not sure what is causing your problem but obviously the first point to clear up is are you using version 10?
Following on from that I notice that, in addition, you also back up to an external HDD so my second question is are you also backing up (or trying to) to a secure zone on your primary hard drive?
If you haven't got v 10 you clearly need it and I can't see any real point in backing up your system twice.
I back up regularly to a separate drive so I have never actually used secure zone but whenever I have had any queries I use the Acronis Forum which can be found here click here They are very good indeed with lots of input from Acronis staff.