Cannot Register for Windows Developer Program

In the Parter Centre, I'm trying to add the Windows Developer Program to the Programs my organisation is enrolled in. I have to log in again with my personal account which goes OK, I'm presented with a list of programs which I'm already enrolled in and may join, at the bottom of which is a further list under the title 'Other areas', which includes Windows app developer, but says 'To enrol in one of the following programs Sign in again using your personal account.'
I sign in again but get an internal server error message
"Value cannot be null. (parameter 'key')\r\n\r\n"
The only way out of this is going back. When I do, I'm advised to close all browser windows and retry. I do and this time the sign-in works, but takes me back to the original page which invites me to sign in again.
The error is repeatable; if I had the patience I think I could go around the loop indefinitely. I have tried a different browser, no change. I cannot find any help articles or any way of raising a support ticket with Microsoft.
Can anyone help - I have an application I'd like to submit to the MS App Store, but no way of proceeding!