Solved.
The problem was caused by the presence of jQuery before the JSON response in the file /mod/wiziq/locallib.php.
[{"error":false,"data":{"content_items":[{"id":3,"name":"choice","title":"Anketa","link":"https:\/\/moodletest.mup.cz\/course\/mod.php?id=663&add=choice","icon":"
[{"error":false,"data":{"content_items":[{"id":3,"name":"choice","title":"Anketa","link":"https:\/\/moodletest.mup.cz\/course\/mod.php?id=663&add=choice","icon":"
After uninstalling A Virtual Classroom Plugin: WizIQ: mod_wiziq
immediately everything started to work perfectly.
immediately everything started to work perfectly.