Cannot submit form due to new backend validation

[SilviaAmAm]

Product versie / Product version

2.6.0

Omschrijf het probleem / Describe the bug

Create a form with:

• An optional text field with multiple = True
• A date field
• Something else, but I am not sure what yet

Try to submit: this shows that the call did not validate on the backend. The POST to _complete gives in the invalidParams:

 [
		{
			"name": "steps.0.data.textField.0",
			"code": "null",
			"reason": "Dit veld mag niet leeg zijn."
		},
		{
			"name": "steps.0.data.date",
			"code": "invalid",
			"reason": "Date heeft het verkeerde formaat, gebruik 1 van deze formaten: YYYY-MM-DD."
		},
		{
			"name": "steps",
			"code": "invalid",
			"reason": "None"
		}
]

Stappen om te reproduceren / Steps to reproduce

See form with slug analyse-design-shortcomings on our test env

Another case that has issues, DH form "Beroep op vrijstelling schoolinschrijving"

Validation errors from complete call:

{
    "type": "http://localhost:8000/fouten/ValidationError/",
    "code": "invalid",
    "title": "Invalid input.",
    "status": 400,
    "detail": "",
    "instance": "urn:uuid:3a2e9ce0-0079-4cfe-bbf2-9f90a0e187c0",
    "invalidParams": [
        {
            "name": "steps.0.data.anoniem",
            "code": "required",
            "reason": "Dit veld is vereist."
        },
        {
            "name": "steps.0.data.persoonsgegevens",
            "code": "required",
            "reason": "Dit veld is vereist."
        },
        {
            "name": "steps.0.data.adresgegevens",
            "code": "required",
            "reason": "Dit veld is vereist."
        },
        {
            "name": "steps.0.data.voornaamPrefill",
            "code": "blank",
            "reason": "Dit veld mag niet leeg zijn."
        },
        {
            "name": "steps.0.data.achternaamPrefill",
            "code": "blank",
            "reason": "Dit veld mag niet leeg zijn."
        },
        {
            "name": "steps.0.data.huisnummerPrefill",
            "code": "blank",
            "reason": "Dit veld mag niet leeg zijn."
        },
        {
            "name": "steps.0.data.straatnaamPrefill",
            "code": "blank",
            "reason": "Dit veld mag niet leeg zijn."
        },
        {
            "name": "steps.0.data.plaatsPrefill",
            "code": "blank",
            "reason": "Dit veld mag niet leeg zijn."
        },
        {
            "name": "steps.0.data.contactgegevens",
            "code": "required",
            "reason": "Dit veld is vereist."
        },
        {
            "name": "steps",
            "code": "invalid",
            "reason": "None"
        },
        {
            "name": "steps",
            "code": "invalid",
            "reason": "None"
        },
        {
            "name": "steps",
            "code": "invalid",
            "reason": "None"
        }
    ]
}

[sergei-maertens]

@SilviaAmAm with the linked PR I was able to complete the form after exporting it from our test env

I can't reproduce this error bit:

		{
			"name": "steps",
			"code": "invalid",
			"reason": "None"
		}

edit: just saw it with another form (description updated). I suspect this is present as soon as there are validation errors for a step, need to investigate further!