Payment period - not working - users can't make any payment to upgrade

Hi, I just got a notification from an user, that she can't upgrade her subscription. I first thought it's because I still have some updates available. After updating all the plugins and the themes, the problem persists. I've created a test user and site, tried to upgrade, but after logging into the main website to the pro page, I only have the option to choose the Payment period. From the dropdown I have the three options, but no matter which one I choose, nothing else happens.
Can you, please, tell me what's wrong?

Thank you

Marius