There are two potential root causes for this issue, either there is another piece of software using jgroups (standalone/WildFly/JBoss EAP/Infinispan/etc) on the network using a different version of JGroups or something completely unrelated is using the same multicast IP/port. The former typically happens when users use multicast for discovery with no authentication nor encryption but use the same UDP multicast address.

Since you are using static discovery, finding the root cause should be easier. You should inspect all running Java processes for potential conflicting version but most likely, you need to examine what else is sending packets on this address (e.g. using Wireshark).