Can't do zero off-set on 4iiii Precision powermeter

Has anyone had this problem and has been able to solve it?

I can't do zero off-set on my 4iiii Precision Ultegra 6800 powermeter any more. It's been working fine since I bought it about 14 months ago or so but on a ride a few days ago it worked fine until I did a sprint, after which it was...