Have you previously entered into or modified the MEBx of the HP dc7800 system? Changing of the default password (which is typically "admin").
The HP OpenView configuration service is based on Intel SCS. By default, when the setup.bin file is created - the old password is "admin". If the MEBx has been modified - the process will fail due to failed authentication as you've experienced.
Take a look at http://communities.intel.com/docs/DOC-1210 - the article talks about the USBfile.exe utility. This allows you to create and view setup.bin files with the ability to customize\change the old password from something other than admin.
If unsure if the MEBx has been modified - do a "Ctrl-P" after POST and before the OS boot loader starts. You can also check the HP BIOS to check whether the AMT properties get displayed. Once you've entered into the MEBx - try logging in using "admin" as the password. Per your notes\indications below - this should fail. If unsure what the current MEBx password is - a CMOS clear will reset (e.g. remove the CMOS battery). I realize this works great for a single system, yet not desireable for a larger quantity of systems.
Let us know if that helps.