Do you have another robot of the same type ?
If so, you can try to swap the controller cabinet to see if the problem is in there.
ofcourse, use the original cpu-board, and be sure that the boards have a working battery connected all the time.
Believe it or not, it was an allen bradley Point I/O module for the end of arm tool in the arm. When inspecting cabling for an ethernet fault I wiggled the RJ45 cable plugged into the Point I/O module and was able the generate the DTERR fault. I was able to do this several times. Changed out the Point I/O module and everything is back to normal.