ModBus Timeout Error
In some cases staring a motor or any high-power devices generate noise on grounding signal. If you don't have good chassis ground and shielding this affects your signal.

for debugging:
1) run your controller without heavy load, just check controller with a led or any small load, after setting that led on/ off, if you have a problem then you have a problem with code and implementation.

2) if you don't have a problem in the first step, you should check the grounding and chassis ground

recommendation:  if you work with high voltage load, for safety it's better to use an isolated USB_to_485 module.