ERROR TcpInputProc - Message rejected. Received unexpected message of size=369295616 bytes from src=xxxx:xxxx in streaming mode. Maximum message size allowed=67108864. (::) Possible invalid source sending data to splunktcp port or valid source sending unsupported payload.
Can you check that you are not sending e.g. some tcp feed to splunk-tcp port which are expecting S2S protocol. There should be separate ports for other than S2S traffic defined one per different protocols.
Have you defined on both side (UF and Indexer) that port the same way and also use the same certs etc?
Have you a separate port for splunktcp or are you using only splunktcp-ssl? You cannot mix that traffic to one port.