If a log needs to be written and logging to the database fails, the log is written to disk instead. For example, this might occur if the database is completely inaccessible. By default, logs are written to the /uploads/logs directory. To change this, add the following line to your constants.php file:
define( 'LOG_FALLBACK_DIR', '{root}/directory' );
Changing the "{root}/directory" place to be the location on your server where the logs should be written. If you include {root} then that will be replaced with the directory to where IPS Community Suite is installed.