If you have YII_TRACE_LEVEL constant defined in your entry script you need to remove it.
Here is what found: http://www.yiiframework.com/doc/guide/1.1/en/topics.logging
Quote:
Yii supports logging call stack information in the messages that are logged by calling Yii::trace. This feature is disabled by default because it lowers performance. To use this feature, simply define a constant named YII_TRACE_LEVEL at the beginning of the entry script (before including yii.php) to be an integer greater than 0. Yii will then append to every trace message with the file name and line number of the call stacks belonging to application code. The number YII_TRACE_LEVEL determines how many layers of each call stack should be recorded. This information is particularly useful during development stage as it can help us identify the places that trigger the trace messages.