What does this error mean?

As the error message states, an unexpected exception was caught in the InboundMessageHandler class.

Why does this error occur?

This message occurs when the fatalExceptionCaught method catches an instance of the InvalidLegacyProtocolMagic exception. This means the MAGIC_PROTOCOL prefix value does not match the proper value of 0xCA552DFA (3394579962). 

How do you fix this error?

Because this is a mismatch in message types, it is likely due to misconfiguration between nodes. If this message is occurring during an upgrade, you can safely ignore until the upgrade is complete. This should end the messages. If the error still occurs, verify that all nodes are configured equally around messaging.