ER
A couple of comments as I've used that set up for testing - and its great

1) If you don't use your own bat file and CFG and instead just amend the medieval2.preference.cfg file - it lasts for one go and then the game will overwrite the medieval2.preference.cfg file and effectively wipe out any extra lines you added.

2) Regarding the use of

[log]
to = logs/system.log.txt
level = * trace

and

level = * error

If you put both in it will use the last one eg

[log]
to = logs/system.log.txt
level = * trace
level = * error

works with just error on.

Just in case like me you get over tired and just can't quite remember what the other command was.