You will need to set (temporarily!) a value for the config variable $courseFiles{logs}{activity_log} in webwork2/conf/defaults.config so that the activity log file will be created/written. (See the warning about it getting huge in the config file - so don't leave it live all the time.)
It could be that log management tools could help manage the size of the file, if you find that it provides data you would want to have available for a certain duration.
Nathan