Only recently I stumbled across the same problem and managed to find a solution. Upon turning on the debugging in Moodle, I received an explanation stating that a default value for the generalcomment (and subsequently teachercomment) field was missing. Since I was unable to set the default values in the relevant MySQL table, I added the following code to assess.php and locallib.php (in the mod/workshop folder):
$assessment->generalcomment = "";
$assessment->teachercomment = "";
The code was added before the following statement in assess.php:
if (!$assessment->id = insert_record("workshop_assessments", $assessment)) {
error("Could not insert workshop assessment!");
}
and before the statement below in locallib.php:
if (!$assessment->id = insert_record("workshop_assessments", $assessment)) {
error("List Student submissions: Could not insert workshop assessment!");
}
http://moodle.org/mod/forum/discuss.php?d=52978&parent=261469
The above worked and the workshop module does not report errors any more.