When deleting a response:
Debug info:
Error code: incorrectaccessdetected
Stack trace:

line 463 of \lib\setuplib.php: moodle_exception thrown
line 667 of \mod\survey\classes\view_manage.class.php: call to print_error()
line 65 of \mod\survey\view_manage.php: call to mod_survey_submissionmanager->prevent_direct_user_input()

Output buffer: <br /> <b>Notice</b>: Trying to get property of non-object in <b>M:\server\htdocs\moodle27\moodle\mod\survey\classes\view_manage.class.php</b> on line <b>598</b><br />