A survey may not be submitted because an answer GUID does not belong to a question. That might lead to an HTTP error 400 (Bad Request).
Single and multiselection questions have predefined answers, each of them internally identified by a GUID. If the answer to that kind of question is a GUID that does not belong to the question's answer set, it is rejected.
This is an example of the issue:
The answer GUID provided in the optionGuid parameter is not among the valid options for that question.
Make sure the answer GUIDs are valid for all questions.
After fixing the issue, submit the survey again.