A theorem:
When h264 (mov) crashes and h264 (mp4) does work (or vice versa), then there could be a bug with the graphics drivers. You could switch on mimoLives debug mode to switch on/off the hardware encoder, to test if this helps (as a workaround):
Just type “Debug” at the help window. Activate the mode at the so called “Debug preferences”. Quit mimoLive and reopen it. You’l get a new menu called Debug.
I hope this helps to proceed with mimoLive. If there is realy a bug, please report it to Apple: https://feedbackassistant.apple.com