This issue is specific to G3 CNC with C-Axis (Spindle Positioning) option.
The asterisk in the middle of the screen indicates that the RS232 communication cable is disconnected or defective.
This cable runs from the back panel of the CNC to the spindle drive in the electronics cabinet.
The connector is a DB-9 labled "RS232".
Rear panel of G3 CNC showing location of RS232 connector.