It had to do with this problem:
https://moodle.org/mod/forum/discuss.php?d=366025
Still, I don't understand why the mod Questionnaire needs an empty 'username' column. What happened there?
In reply to Richard van Iwaarden
Re: Field 'username' doesn't have a default value
by Chris Chapman -
Got the same issue (linked in your reply), going from Moodle 3.0-3.3 was this ever fixed by developer in a new release of the plugin?
The 'username' field was renamed to 'userid' back in questionnaire release 3.3.0. What version of questionnaire are you using?