If you are using the HP software as stated, remove it, the SCSI card, and
power down.
Boot the PC and let Windows detect the SCSI card. It will then detect the
scanner. Let it install the generic HP 6C driver. If it fails the "Test Scanner"
with the same error, you are out of luck.
Just a FYI, I have worked on hundreds of these scanners (I use one myself).