Have you tried "swapping"the wires from the controllers to the motors? If it does it -- opposite(ly) maybe a set up problem -- If still the same (A side) I'd guess the motor.
That won't get you to testing the Lag - but will confirm that it is a motor problem and not a steup in the software.
(Also, it might point to a physical binding on the "Ä" side.)
Hope that gets you a start - surely others will have a good idea or 2 as well.
Cheers - Jim