A "Failed to copy from file" error is caused by one of 3 things:
1. A configuration issue in the server or client (or both)
2. A lack of a shared directory between the server and client
3. A job that took longer to execute than the document timeout setting in the adminui
I suggest 3 steps to diagnose this:
1. Review the other thread in this forum that discussed similar symptoms and solution:
http://www.adobeforums.com/cgi-bin/webx/.3bc0ff0f/132. Deploy the Assembler sample web application (in C:\Adobe\LiveCycle\Assembler\samples\build\prebuilt) and use it for a simple 2-document assembly to verify that your installation is working
3. Determine the differences between the sample and your actual client program.
Hopefully those 3 steps will illuminate the problem. If not, then post the details on what you are doing that is different from the sample and someone on this forum may have an idea for you.