I haven't been following this very closely so I may be stating the obvious
(or totally off base, take your pick :-) but it looks like you may be
getting a serial line interrupt before the kernel has configured the dca
(serial) device.  If the interrupt handler hasn't been registered, this
could cause bad things to happen (though it should catch unexpected
interrupts more gracefully I would think).  This might not happen with the
graphics device as console since the boot ROM would then not be configuring
the serial port (as it must when using the serial console).