What does the "MIMO Event MIMO: ..." log message mean ?

Cable modem log files may serve as a useful troubleshooting tool, provided one can decrypt the various types of messages. You may notice lines in your cm log file of the following type:

MIMO Event MIMO: Stored MIMO=-1 post cfg file MIMO=-1; CM-MAC=....;CMTS-MAC=....;CM-QOS=1.1;CM-VER=3.0;

DOCSIS MIMO stands for MDD IP Mode Overide. On a D3 cable modem system, the CMTS sends a packet called an MDD (Mac Domain Descriptor) that tells the modem specific details about the cable plant, they determine whether the cable modem attempts to boot up in IPv6 or IPv4 mode. Some of those are TLV 5.1 and 5.2. You get those notices if the mode is not set in the modem config file, or if there is a difference between the current operation mode and what is transmitted in the MDD.

Generally, this should not affect operation of the modem, as it's just a notification. The MIMO event is one indicator that the modem has reset.