This has happened many times since I've updated to the latest version of Media Encoder 14.3 (Build 39). I'd place a video in the Queue and hit Play and it will start the first frame and then freeze my entire machine only letting me move the mouse, but can't click anything or use my keyboard to try to force quit or anything, and the only way to get out of it is to force shut off my iMac and restart the machine.

I've had this issue happen to me over 10 times already!