Cause : BU DC link difference comes when DC link voltage difference detected by the branching unit. The branching unit is an optional module that allows the ABB ACS880 drive to control multiple motors simultaneously. Each motor connected to the branching unit has its own DC link

An AE0C fault code in your ABB ACS880 drive indicates a  branching unit DC link voltage difference. This fault occurs when the DC link voltage between the main drive unit and the branching unit (if you're using one) differs beyond the acceptable limits.

1. Check DC fuses.
2. Check converter module connections to DC link.