Description of problem:
I use simplesurvey version 1.0 with CMS 6.0.1.

in Response.pdl :
object type Response extends ACSObject {
......
    component Answer[1..n] answers = join ss_responses.response_id 
                                       to ss_answers.response_id;
......

constraint 1..n is wrong because answers can all be optional.

Did this issue not appear with CMS 5.2 because of constraints not
rigorously enforced in 5.2 ?

The solution is to change [1..n] by [0..n]

Version-Release number of selected component (if applicable):
simplesurvey 1.0
CMS 6.0.1 (must be the same issue with 6.0.0)