GSK980TDc and GS3048Y user:
M20 and M21 do not seem  to be set up properly. The spindle clamp activates breifly whenever M05 is executed, and the spindle clamp also activates breifly when M20 is executed while spindle is running.

If M20 is executed while the spindle is stopped, the spindle
is not clamped but I can not start the spindle again until M21 is executed.

CNCmakers Support:
The M20 and M21 must have been reversed, please exchange the control signals of two solenoids.
M20 is spindle clamp, M21 is spindle loose, the M20 must be executed when the spindle is stopped, please kindly check the wiring.