I'm having a pretty specific issue with Questionnaire on Moodle 3.5 and 3.6 using Nginx and PostgreSQL on Gentoo machines - I've been able to replicate it on two machines so far. Both machines are running the latest version of Questionnaire - 2018121000.
To replicate the issue:
1. Create three courses - we'll refer to them as Questionnaire Home, Questionnaire Outpost 1, and Questionnaire Outpost 2.
2. Make a questionnaire in the Questionnaire Home course, set it as a Public questionnaire. Give it at least one question.
3. Make a questionnaire on the two "Outpost" courses, using the content from the questionnaire on the "Questionnaire Home" course.
4. Submit responses on the two questionnaires in the "Outpost" courses, so that you end up with two responses visible in the "Home" questionnaire.
5. Attempt to use the "Delete ALL Responses" control in the "Home" questionnaire to delete the responses. You'll get a blank white screen, with no error messages in any log I was able to find, even with debugging at maximum.
You can get around the error by submitting at least one response in the "Home" questionnaire - then "Delete ALL Responses" will work. This leads me to think that the error happens when you try to use "Delete ALL Responses" in a questionnaire that has responses, but all of those responses come from questionnaires other than the one you're using the "Delete ALL Responses" control on.