After an update from moodle 1.9 to 2.x in some circumstances the field size of mdl_log.action is to small.
In our especially case the size of this field is 15 but it has to be 40.
If the quiz-module tries to write a log entry while a user is continuing a quiz the action value is "continue attempt".
This value has a length of 16 chars and throws a dml_write_exception.
I think there are more cases than mentioned here.