Initially we set up our payment templates with two options - PayPal and Manual Payment. Neither was set as default with the default buttons, it just happened that PayPal came first with Manual Payment second. Both options have worked for us in all cases for over a year.
Recently we decided we wanted to make Manual Payment appear as the default option - Rather than delete all payment options and add them back again across 20 templates so that Manual was first then Paypal, we simply clicked the DEFAULT button for the Manual Payment option in each template. This now makes the Manual Payment option show up by default in the drop-down, but whether you drop-down and select it, or just leave it as the default option, the form responds with the error "A required field has not been completed" and you cannot submit the form. If however you select PayPal from the drop-down, it works.
it is as if selecting the default button on this option has negated this option from being active - leaving you only able to select PayPal or the field is blank.....