Noticed this a couple days ago. I'm assuming they finally shut the Nokia account servers off. Might be stuck using the free version until they announce something on May 19th.
EDIT: Found this. Nokia account was discontinued on April 24th, so there is currently no way to sign in to MixRadio
https://forums.windowscentral.com/e?...token=F22HBZyJ
◦Nokia Account will be discontinued on 24 April 2015. In its place, you can now create a Microsoft Account at
http://login.live.com. We apologize for any inconvenience this may cause.
Based upon what I'm seeing personally, devices that were already signed in seem to be staying that way, but if you're setting up MixRadio on a new device, or after a factory reset, you're out of luck.