ModTracker error when adding comment

[slechaix]

Hi,
does someone have this error and corrected it ?

I use modtracker to track any change on modules like Tickets, it works well when modifying any field but it's not working as soon as I insert a comment.

Code:

Uncaught SyntaxError: JSON.parse: unexpected character at line 2 column 1 of the JSON data
    showhistory http://cbprdtest/modules/ModTracker/ModTrackerCommon.js:139
    jQuery 6
    showhistory http://cbprdtest/modules/ModTracker/ModTrackerCommon.js:134
    <anonymous> javascript:ModTrackerCommon.showhistory('223451'):1
ModTrackerCommon.js:139:27
    showhistory http://cbprdtest/modules/ModTracker/ModTrackerCommon.js:139
    jQuery 6
    showhistory http://cbprdtest/modules/ModTracker/ModTrackerCommon.js:134
    <anonyme> javascript:ModTrackerCommon.showhistory('223451'):1

httpd error log:

Code:

PHP Fatal error:  Call to a member function getFieldLabelKey() on string in /var/www/html/corebos/modules/ModTracker/core/ModTracker_Field.php on line 25, referer: http://cbprdtest.viti.pf/index.php?module=HelpDesk&parenttab=ptab&action=DetailView&record=223449

Thanks for your help