That assertion is configured to fail whenever the virtual machine's CPU encounters a triple-fault. Our assumption is that if you are running with debugging enabled, a triple-fault is an event that you're likely to want to capture and debug, so it's wired up to trigger a virtual machine monitor panic by means of raising an assertion failure. It won't do this when the VM is configured with Gather debugging information set to anything other than Full.
With monitor.suspend_on_triplefault set, this should produce a pair of files named debug.guest (which is a trimmed-down .vmss file, not suitable for restoring the state of the whole VM, but containing enough CPU state to enable debugging) and debug.vmem (containing the state of the VM's memory). Are those files not being produced? Those two files should be suitable for use with vmss2core, which should be able to produce regular ELF corefiles from the VM's state at the time of the triple-fault.
Cheers,
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Darius