Hi! I've finally solved the mystery of the "answer id" message being displayed in front of multiple choice questions in the Lesson module.
The problem (and the solution) is the same in versions 1.5 and 1.5.2
In a given theme's config.php file, you must have $THEME->modsheets = true;
but this is not enough... you must also have $THEME->standardsheets = true;
This is not clear at all from the comments in the config.php file.
Another solution - actually better if you are doing heavy theme customizing is to move all the contents of those mod styles and blocks styles files into one of your theme css files...
All the best,
Joseph