In the Questionnaire, go to feedback. Use feedback per section. Create a label with a long name. 

Error will popup 'Error writing to database'. No further information, unless you will turn on debugging and see the following:

 
Debug info: Data too long for column 'sectionlabel' at row 1
INSERT INTO mdl_questionnaire_fb_sections (surveyid,section,scorecalculation,sectionlabel,sectionheading,sectionheadingformat) VALUES(?,?,?,?,?,?)
[array (
0 => '2',
1 => 3,
2 => 'a:0:{}',
3 => 'Zet doelstellingen om in activiteiten voor een plan',
4 => '',
5 => '1',
)]
Error code: dmlwriteexception
Stack trace: * line 497 of /lib/dml/moodle_database.php: dml_write_exception thrown

• line 293 of /lib/dml/moodle_read_slave_trait.php: call to moodle_database->query_end()
• line 1482 of /lib/dml/mysqli_native_moodle_database.php: call to mysqli_native_moodle_database->query_end()
• line 1528 of /lib/dml/mysqli_native_moodle_database.php: call to mysqli_native_moodle_database->insert_record_raw()
• line 101 of /mod/questionnaire/classes/feedback/section.php: call to mysqli_native_moodle_database->insert_record()
• line 149 of /mod/questionnaire/fbsections.php: call to mod_questionnaire\feedback\section::new_section()
  --------

So the label name has a maximum number of characters, but the error doesn't show it and the form field allows past the maximum.

My idea would be to have a maximum number of characters in the form field 'label' and thus avoiding the database write error.