What could cause a VirtualBox guest to enter a VBOX_E_INVALID_VM_STATE

I'm running several VirtualBox servers on CentOS, with multiple Virtualbox guests on each.

Once every few days/weeks, I'd have a guest stop responding, and though it shows as running, it is actually in a VBOX_E_INVALID_VM_STATE, and only the following will abort it:

VBoxManage startvm "<guest-name>" --type emergencystop

I'm trying to determing the cause for my guest machines entering VBOX_E_INVALID_VM_STATE.

Are there any known situations that will cause a guest machine to enter VBOX_E_INVALID_VM_STATE, or can you suggest ways to debug this issue?

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