I was speaking to @Jack Kitterhing via chat and regrettably we weren't able to come up with a quick solution, so I'm posting to try and determine a fix for my problem. I recently updated my membership plugin with the purchase of a WPMUDEV license and this resulted in my registration page no-longer being functional.
Upon attempting to register, the page refreshes, whereas it formerly would register the user and process them to the account payment page.
I've deactivated all modifications and the page is now using the "subscriptionform" shortcode as per @Jack Kitterhing request.
Any help would be greatly appreciated!