The first error you show (communications error) means the plug-in was unable to talk to the PMDX-424. The usual cause of this is either electrical noise or a loose/faulty USB cable. Have you seen his error at any other time (like normal motion of the Z axis)? The 424 has an isolated USB interface, which *should* make the USB less susceptible to electrical noise, unless you have wiring somewhere that connects various grounds (the PC's ground and either of the 424's isolated grounds) which then defeats the benefit of the isolated USB.
Standard steps to help keep motor noise (if that is what is causing this) out of the system include: Make sure that the USB cable is nowhere near any motor wiring (power supply, motor leads, etc.). Also make sure that all limit switch wiring is as far from the motor wiring as possible.
The second error I have no idea. If you did not exit Mach4 and unplug the PMDX-424's USB port after the comms timeout, then all bets are off and this may have been due to some residual state in the plug-in or device that wasn't cleaned up (a total wild a-- guess here).
