They couldn't be bothered to come up with a unique acronym.
DOCSIS MIMO stands for MDD IP Mode Overide. On a D3 cable modem system, the CMTS sends a packet called an MDD or Mac Domain Descriptor that tells the modem specific details about the cable plant. Some of those things are TLV 5.1 and 5.2. They determine whether the cable modem attempts to boot up in IPv6 or IPv4 mode. If there's a difference between the current operation mode and what is specified in the MDD (this error often comes from a difference in the modem config file, and what is being transmitted in the MDD). It shouldn't affect operation of the modem, as it's just a notification.