The first step is to take all load off motor. The ERR20 indicates the motor is not getting where requested and is either missing to many steps or required to issue too many additional pulses in an attempt to get where it was requested to go. Several things could be at play. Make sure the encoders are indeed wired correctly, those are differential signals with A, A/ , B, B/ and all four signals must be present for these encoder signal to be understood correctly.

Russ