With friend who bought a 6545, we had a problem like that. That was an electronic noise problem on emergency stop.
To diagnose, you should first try to program Mach3 to ignore emergency stop (red mushroom). Then start the spindle.
If that works, reenable it, and open the cover of the controller. Try to start the spindle. If it"s ok, that's really a noise problem.
To solve it, rewire the emergency stop with shielded cable, and route it as far as possible from the CFD board.