Error 'form[fieldName] :  is invalid' while submitting multiple checkboxes
I'm not sure if it can make a difference, but you should try to remove the white space between the two array in the name attribute.

Then I think you may have to format the date value with the date filter.

&lt;label&gt;
    &lt;input :class="{ error: error['formDate'] }" type="checkbox" name="fields[formDate][]" value="{{ date.value|date('Y-m-d') }}"&gt;{{ date.label|t }}
&lt;/label&gt;