Cause : Safe torque off comes when safe torque off function is active, i.e. safety circuit signal(s) connected to connector XSTO is broken during start or run.

The fault code 5091 in your ABB ACS880 drive indicates a  Safe Torque Off (STO) active condition. This means the drive has detected a deliberate shutdown of the motor torque due to an activated Safe Torque Off function. Check safe torque off circuit connections.