Developer from Greece here.
Having the same issue as everyone since April. Tried 3 different credit cards. 2 from Greek banks and one from Citibank US.
My order gets canceled after one day, with no notification whatsoever.
I have made multiple contacts with Apple support, and they always reply with automated messages, ignoring what I wrote.
I have taken all the suggested actions like:
Login in to the Apple Id with an Apple computer.
Add all credentials (addresses etc...) and credit card details there.
Login to iTunes with same Apple Id.
Login to App Store with same Apple Id.
Made a purchase from App Store using the same credit card as in my account.
Updated Safari, cleared all caches.
Logged in to safari Apple Id and checked the validity of payment method and credentials. It was correct.
Procedded with program enrollment and get canceled after one day.
Good luck to everyone!