As I know the slotted disc is for feedback to the lenze drive via the controller board (the large board with many connectors) I don't think that rigid tapping is possible with this machines, and you don't have as much torque as you need for tapping. The motor has with the small toothed belt gear a maximum of 2Nm. That is enough only for M6 in Alu or M4 in Steel.

I hope I come in the next days to the machine, then I make some pictures and a little drawing of the
part with the motordriver. Why don't you use a Multimeter to see if the voltage is changing from your controller board. If yes, then something in the harness from controller to driver is not ok.