Error in optical system.
Turn the ScanSnap off then back on, and try again.

Cause

The light intensity of the lamp inside iX500 is extremely low. It may be caused by contamination of the glass inside iX500, a dead lamp, or a failure of iX500.

Resolution

Open the ADF top section of iX500 to remove the document remaining in the ADF.
Turn iX500 off then back on.
Clean the glass inside iX500 if it is dirty.