I've almost the same problem.
I've 4 credits and 4 single pictures I want to buy.
I go on "licence" but it shows me "error". Unable to purchase the picture. However, I can still download previous pictures.
I tried on two computers, CC, Chrome and Firefox.
Since yesterday it doesn't work.
Thanks a lot.
So my initial 4 credits don't work if I understand.
Only while the problem was happening. It's been resolved.