That packet that has an unknown opcode is a spell file checksum that is used on Live. It is something the client sends and I think the only server that actually uses it currently is P1999.
It is normal to see that error. We don't need to handle all unknown opcodes, so we don't bother adding code to support them. Would be nice to have them all known and handled at some point, but it isn't anything game-breaking, so it is low priority.
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