This error was caused by maat’s sensitivity to the format of its input and a typo I made in the git log to generate maat_evo.log.
The command has to be exactly the following with the colon followed immediately by [%h], without any whitespace:
git log --pretty=format':[%h] %aN %ad %s' --date=short --numstat --before=2013-11-01 > maat_evo.log
Then, every non-blank line of the log will start with either a number or an opening bracket and the matt summary will succeed.