It seems that Questionnaire 3.5.2 (only ?) is not compatible with (earlier?) Moodle 3.5.2 builds (before Moodle version 2018050800) that contain the older admin privacy tool (2018051415 instead of 20180514114 ?
Unfortunately, I cannot upgrade my site to 3.5.3 yet, while my tests of Questionnaire 3.5.2 did not detect errors, so I recently also upgraded the production, and - as I have now discovered - I am faced with a non-fuctional Plugin privacy registry page, which goes blank while the debugger only throws this error:
Fatal error: Interface 'core_privacy\local\request\core_userlist_provider' not found in /ocfs2/www/owko/mod/questionnaire/classes/privacy/provider.php on line 30
I wonder if any simple backward compatibility could be provided. I wish Moodle had stopped me from upgrading if it was know 2018050800 was the core version required for the newer Questionnaire versions to work...