Sounds like a timeout to me. We saw this once before on a client where Bootwiz was taking too long to run. Give this a shot.
Navigate to program files \ Altiris \ Deployment \ Web
Find the Web.config file and edit it.
Find the line that reads httpRuntime maxRequestLength="10000"
Before the closing /, add executionTimeout="1200"
Save the file
Restart IIS
Try again.
That should increase the timeout from the default 5 minutes to 10 minutes. For one customer, that was enough.
Let us know.