the expression pedals, IIRC from taking the back off of my fcb1010 a while ago, are optical. the mechanism uses a graduated curve of transparent plastic attached to the pedal, which moves back & forth through a photo-transistor/LED pair.
something might be occluding the PT, or maybe some contaminant has built up on the transparent plastic part, or else the PT is "going off"- becoming unstable through noise or just beyond the limits of the unit's ability to calibrate it. taking the back off the board & inspecting the assemblies in question would provide an opportunity to clean them of any ingress, but then a visit to a behringer dealership would have to be next on the list, I'd say.
I have a number of fairly sophisticated walkmans that use PT/LED sensors in their logic circuitry, that have failed in this manner.
just a thought.