Also, someone might have turned on the 'Instant validation' admin setting (recommended) but it might be failing. Look for JavaScript errors in your web browser.
Dear all,
thank you for your answers and sorry for the late response. The moodle-admins at my university told me that they had to adjust a setting in the "styles.css" for STACK [https://github.com/maths/moodle-qtype_stack/issues/480]. Now it works.
Martin