I messaged customer support asking what DRM they used and why could I play preview tracks but not full ones - this is the answer I received:
"Dear Customer,
Thank you for contacting Napster Customer Support.
I understand your query and issue. I apologize for any inconvenience caused. Sure, I'll help you with this.
I see an Active Napster account.
I'm sorry to inform you that Microsoft has removed DRM content since July 14, 2017. So none of the downloaded tracks will work with the Napster application due to the removal of the DRM and the MP3 player will not work either because Removing the WMDRM feature from Windows 7, 8, 8.1, and Windows 10 (1507) version for the Napster V6 PC client.
The detailed analysis of the data showed that this functionality has very little success and it doesn't merit keeping it running. So we made the difficult decision to withdraw it. However, Napster plans to focus more on other great features and integrations that will be added in future updates.
However, you can upgrade your Windows to the latest version, and download the PC application from Windows 10 that allows you to download songs for offline use. But downloaded songs can only be played in the application.
You can also use the Napster app on Android, iPhone, and Windows phones to download songs for offline use.
And regarding the Browser issue,
As of now, the Napster website can be accessed on Firefox, Chrome, and Edge. safari. And we are working with our development team to make the Napster website work on all the browsers.
I hope this helps you.
If the issue still exists, please write back to us with the status of the issue to assist you further.
If you have any other questions or clarifications, please reply to this email, we will be glad to assist you."