Hey Ricon,Yep can confirm this! If you have an old account then you have to create a new account... then you can checkout in the marketplace. I had a old account from 2011 and could not checkout in the marketplace, now I register a new account and it is all working just fine.
I repeat - do not create a new account in order to fix this issue! At one point, we might ban accounts that we find to be duplicates and it will create a big mess.
Please contact marketplace[at]cryengine.com for help if you are having issues checking out or registering for the marketplace in any way. The same goes for being unable to download or install purchased assets!
Our support will help you fix your account and get your assets. Thanks for your understanding.