$ sudo -u www-data php admin/tool/task/cli/schedule_task.php --execute='\core\task\grade_cron_task'                                                                                           

[PID 29648] Execute scheduled task: Background processing for gradebook (core\task\grade_cron_task)

Default exception handler: Exception - Class 'grade_grade' not found Debug: 

Error code: generalexceptionmessage

* line 67 of /lib/classes/task/grade_cron_task.php: Error thrown

* line 157 of /admin/tool/task/cli/schedule_task.php: call to core\task\grade_cron_task->execute()

!!! Exception - Class 'grade_grade' not found !!!

PHP Fatal error:  Uncaught Error: Call to a member function real_escape_string() on null in /var/www/html/lib/dml/mysqli_native_moodle_database.php:1112

Stack trace:

#0 /var/www/html/lib/dml/mysqli_native_moodle_database.php(1135): mysqli_native_moodle_database->emulate_bound_params('UPDATE lock_db\n...', Array)

#1 /var/www/html/lib/classes/lock/db_record_lock_factory.php(179): mysqli_native_moodle_database->execute('UPDATE lock_db\n...', Array)

#2 /var/www/html/lib/classes/lock/lock.php(102): core\lock\db_record_lock_factory->release_lock(Object(core\lock\lock))

#3 /var/www/html/lib/classes/lock/lock.php(116): core\lock\lock->release()

#4 [internal function]: core\lock\lock->__destruct()

#5 {main}

  thrown in /var/www/html/lib/dml/mysqli_native_moodle_database.php on line 1112