Dunno how long this has been going on but I can't buy premium (not that I want to). Ceridwen tried to buy it a week ago and found that it didn't work so I tried it too. I thought it was just a paypal error but it's still not working now.
After going to User CP > Paid Subscriptions and choosing 1 month. I follow these screens:
After entering my details and logging in I get this error:
I don't know if this is anything that can be fixed from here or whether it's paypal, but it definitely needs looking into.