I downloaded the jbpm-installer and used the ant tasks to install the demo. This worked perfect for me on my win xp developer machine. On our linux test server I allways get the error with "Premature end of file". This occures even with the new 2.1 version you provided. We are using Ubuntu 10.04.4 LTS. My first suggestion was an line ending problem between linux and win because on one os it worked and fails on the other. But I checked the files and they all have linux line endings. The other difference is, that the sever runs remotely at our hoster and it's not working. The working installation is running locally. I don't know if this matters. If it helps you can send me a pm and I can give you access to the linux installation.
I would like to provide you log files, but there is nothing related to the error in the logs. Maybe you can tell me how to log more related to the error?
René