session_start(): Cannot find save handler ‘mm’ – session startup failed in /sites

This error means that your session can not be saved by your session save handler. As you see you are using mm session save handler which stands for Shared Memory.

From php.net:

To use shared memory allocation (mm) for session storage configure PHP
–with-mm[=DIR] .

You probably need to install php Shared Memory extension to use this kind of session handler:
http://php.net/manual/en/book.shmop.php

Default php session handler is file handler which store your session in files on file system. This handler do not need any additional extension.