That exception is most likely being raised here. The relp input accepts a socket connection and then calls the method that throws the exception.
When a RELP client connects I believe it has to send an "open" command (to configure the session) before it can send a "syslog" command (to log a message). The state diagrams in librelp-1.1.0/doc/relp.html suggest that too.
The only code path I can see to get to that exception would be for the first command sent to the input to be "syslog", and that violates the protocol. You have tcpdump data, so what does the stream of commands after the socket is opened look like?
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