I had this problem with Ai5.5. So I upgraded to 6 yesterday. And guess what, the same behaviour. Screen grays out when the actual scan starts and tells me to restart my computer. I did not think OSX could be like this.
( It worked fine in 10.1)
I have a Mac G3 B/W 350 with 256 mb Ram. A adaptec 2906 card with a Polaroid SS 4000 scsi.
You're getting a kernel panic and it's likely NOT down to SilverFast - something about SCSI is messed up. You should find that if you go to the apple mneu and call up About this Mac you can get the Log files for all the crashes.