After tinkering with this form a bit locally, I was able to determine that this error is being caused by an invalid entry in the instance::db-doc-ref column. For the inputs/contact/date_of_birth field, you have instance::db-doc-ref = 0. When I removed that, I was able to submit the form successfully!
We should be able to catch problems like this when cht-conf validates the forms before uploading them to the instance. So, I have logged an issue to add this additional validatation!
(Side note related to form validation: when tinkering with the task_sample_reception form, I did notice that cht-conf reported several validation errors. None of them were related to the error, but you should make sure you are running the latest version of cht-conf to take advantage of the latest validations and fix your forms accordingly! (Validations run automatically for upload-*-forms actions or can be triggered manually via the validate-*-forms actions.))