On the older IMC controllers, this fault was either the Serial cable between the drive and the controller or the Current Sensor located on a power board behind the LGA board. that fault was "Insufficient DC Loop Current". On the iBox, they have relabeled the fault. I have replaced at least 3 of these current sensors in the past. they are a 3 pin soldered sensor. I would go over all of your DC motor and field connections first. Try adjusting you Field forcing voltage a little higher, 10 to 20 volts, and see if that helps before going after that sensor. Like Trust said, change out that serial cable. the part number should be in the back of manual under appendices.

Good luck.