First thing I would check is to see if there is an Unattend.xml baked into your image at C:\Windows\Panther\
If an XML is already there, Windows setup has already accepted a different XML as the one to use, not the one you inject later via Provisioning. If this is the case, you can probably work around the issue by deleting \Panther\Unattend.xml as part of your provisioning template, and then injecting the one you actually want to use.
If the above is not your issue, then for troubleshooting what Setup thinks is wrong with the Unattend, boot a failed machine to WinPE (or connect the HDD to a different computer) and look at the various log files in \Windows\Debug\ and \Windows\Panther\ (including subfolders). Setup logs will tell you what part of the answer file it deemed invalid, but there are different logs for different pieces of the setup process so you have to dig a bit.
Also, if you are injecting passwords or any other data to your unattend.xml through the use of LANDesk variables, make sure that none of the values being injected have XML/HTML escape sequences or tags. A few times I have run into corrupted Unattend files when replacing %variable% if the real text string included </