FWIW, You mentioned in the OP that you added billing information, but unless their account is very very old: billing information is required on sign up, afaik.
It's possible they did something goofy with a card that expired or a prepaid card upon opening the account that's causing the current issues.
Nope, fairly new account, billing added. Looks like they limit the number of projects which can have billing attached to 3 by default. So any created after that just have no billing associated, with no warning. What a Trainwreck.