I have created a test button that should take you directly to the subscription checkout, hopefully getting round the issue we are having.
Please click on the button below and see what the URL for the checkout begins with.
Don't actually place an order, just look for a URL beginning with "recurringcheckout.com". The page should look like this.
If it doesn't work initially, try pasting the address for this page into an incognito browser and see if that works.
Let me know how you get on!