Lego light sensor corrupts encoder readings on Handyboard

I am using the Handy Board (with expansion board) in my university's robotics class.     I have discovered that when I attach the Lego light sensor to one of
the Lego ports, the amount of "clicks" I get out of my encoder on another Lego port halves. When I unplug the light sensor the amount of clicks per rotation is back to normal. Do you know what causes this phenomena? How can I correct this?
Thanx! -Franky
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