I am experiencing the same problem. I have dug through the PXE boot process and
lie_deploy scripts recently, but no idea how to debug the multicast part. Especially, I have no idea what any of the arguments are supposed to do and how the process/sequence is supposed to look.
Have you gotten any further in the meantime? If not, can you tell me how you understand this to work (like, the sequence of things) - I couldn't find any proper documentation at this level of detail yet. If there is, please point me to it, that will suffice.