The Multicast Expansion Table (MET) is used for multicast packet replication. The table gets populated when a valid multicast entry needs to be programmed into the hardware forwarding table of the switch. If the %MLS-2-MCAST_METFULL message is received, it indicates that the multicast expansion table used to store outgoing VLAN information is full.
One common cause is when the data replication application ghost has sessions running in the network and the ghost server is down. The clients then use the multicast service to discover the ghost server. If there is a particular multicast address used by the clients, then that IP address can be blocked using ACLs. However, if the ACL is used to block the IP address, this would also prevent the entry from being pushed into the MET.