Hi,
There's a new EULA that you'll have to accept. The main difference is that we -- rather than Paradox -- are your counterpart. Payments and payment support are still Paradox' responsibilities.
Only the new client will display an error message properly. Ender's client will simply tell you that you have provided an invalid username or password. Relog and sign the new EULA and everything will be fine.
Enjoy!