Did you try plugging it to a computer when it was 'frozen'? (because besides printing the jailbreak message, there's no other feedback, so when you say frozen I'm assuming it really, really was stuck, which, in and of itself, is quite peculiar).
Exactly how far into the boot process does it go? (Because it shouldn't loop for long, after a while it should throw you the "Your Kindle Needs Repair" screen and shut down).
That's the first time something like that has been reported, so I'm curious
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