I run a Mac on one machine and windows on 2 of them. No issues with 640. You can open the debug screen and see if you can see which hard limit is tripping. hard limits are usually the homing switches. By chance have you replaced the Z limit switch? I found an issue with my machines on that switch cable routing.
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Interesting! We replaced every limit switch on the first machine, and the new machine obviously has all new limit switches, and the issue persisted across all of it
Sorry I didn’t tag you in my last reply. Are you trying to jog immediately after initialization? Because we replaced the main board and all of the limit switches in my original machine and then replaced the entire machine I’m essentially on my third machine and it’s having the exact same issue.
Edit: are you on Intel Mac? I have an M1 Pro
If my understanding of the root cause of Duncan’s issue is correct, the problem isn’t hard limits or switches themselves. It’s a bug in Carbide Motion that causes the machine to move to the wrong position when using Rapid Position immediately after initializing, and in doing so, it runs the machine to the limits of its travel. It’s not related to hardware. I’m going to bug the dev team to take another look at this and get it back on their radar.
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I’ve run into this a number of times myself, most recently was just a couple hours ago.
IIRC there is a thread with a method to reliably reproduce it somewhere around here.
Let the devs know that another person has been impacted by this. <insert “there are dozens of us, dozens!” meme 
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*Nice, looks like Flynn mentioned the relevant thread earlier in this post.
Sucks it’s happening, but glad it’s repeatable! Thanks for the response. Hopefully we they get it patched up soon.
Correct, that’s the working theory! Much appreciated:)
Tested on windows today and same issue, failed during machine setup, and on initializing as well.
It’s so confusing that the issue is not the hardware, but some of us have it and some of us don’t.