none -- I have six MLXe running in MLX system mode running 5.4 and 5.7 that work fine (though we are using snmpv2).
This was user error. Turns out I was trying to use both SNMPv3 options. The read/write option needs to be black because it only sends informaiton. Never writes.
There are no error messages showing in the log on the MLX.
If I remove the SNMP client command I see an error in the log rejecting the server, so I know it is reaching there. I do however have this command in so that it allows access.