I have never seen a document that explains the logs on the 3300... if you find one, let us all know.
Other than that, not sure I can be much help... that is a new one to me, but seems it is clearly a network issue and not a phone system issue since the phones and system don't use IGMP to my knowledge (don't confuse it with ICMP, which is for ping), the Mitel is just telling you it sees something wrong. I would go back to the customer's network people.