Here's what you need to do to get it working.
Press F2 to start the Operator Actions form.
Select the job you want to fix.
On the scan tab, set Threshold to zero (0). This disables this setting.
Set Brightness to 128 and Contrast to 128.
On the enhancement tab, tick the Despeckle 1x1 option.
Save those settings.
Repeat steps 3-8 for as many job types that you have setup.
That should fix the problem and you should now be able to scan okay in black & white.