Can't add payment method to pay for Microsoft Store enrollment

I am trying to get enrolled to distributing my Windows app through the Microsoft Store. So I head over to partner.microsoft.com/dashboard and I press the "Enroll in programs" button:

And then the: "Windows, Get started" button: 

Then I login with my personal account, which is required for this program. Then I choose "Windows and Xbox":

Then I'm directed to either enter promo code (which I don't have) or to add a debet or credit card, (which I choose). I press "Add a new payment method" and enter in my card details, along with extra info it's asking for, and then when I hit "Save", I get this error below (But see further below, for the actual error from F12/Developer tools, which seems indicate a different issue) :

Developer tools / F12 has this failing record in there: 

Request URL : 

https://paymentinstruments.mp.microsoft.com/v6.0/users/me/paymentInstrumentsEx?country=dk&language=en-US&partner=payin&billableAccountId=kZYybAIAAAAAAAAA&completePrerequisites=True

Request Method : POST

Status Code : 400 Bad Request

Remote Address : 13.107.246.44:443

Referrer Policy : strict-origin-when-cross-origin

Now, just to make sure, I have went through these additional steps:

• Tried with another browser (Edge, fully updated) - same error.
• Tried from another PC (actually 4 different PCs) - same error.
• Tried with a Macbook - same error.
• Tried from my remote PC in a another country - same error.
• Raised an issue with Microsoft support (this was 5 days ago now) - no response from support, except form "we received your support ticket" email.
• Tried from another personal Microsoft account - same error.
• Tried with a colleague, using their personal account and their PC - same error.
• Tried 3-4 different payment cards - same error.

Anyone have any clue on this error? 

Is the Microsoft Store not a valid route forward anymore, for enrolling new software developers/vendors? Or is this Tenant-based perhaps? I could of course just buy a more expensive code signing certificate and then try to distribute the software through my own website, but that's not the ideal way forward. I really would like to get this working on the Microsoft Store if at all possible. 

Really hope this issue is resolvable...