This one bothered me quite some time. In the function log_exception() there is a string created like this:
log_message('error', 'Severity: '.$severity.' '.$severity.' --> '.$message. ' '.$filepath.' '.$line, TRUE);
You see the double $severity ?? one can be removed like this:
log_message('error', 'Severity: '.$severity.' --> '.$message. ' '.$filepath.' '.$line, TRUE);
