Hi Werner,
Thanks for the update.
$("form:first").submit(); - $(“form:first”) returns the form element as object collection and first form elements you need to take $(“form:first”)[0].
document.forms[0] - document.forms[0] returns the form element directly. Because document.forms returns the object collection and document.forms[0] returns the first form element.
Please get back to us if you require any further assistance.
Thanks,
Christo