I heard back from the Napster folks.
It’s a Windows 10 Anniversary Update issue – the most recent version removed support for WMDRM. Napster is working on a Windows 10 app that will allow download and offline playback, but no ETA on when that will be available.
Here’s the full reply from Napster.
Please note that customers who have installed Windows 10 Anniversary Update on their PC will no longer be able to use the Mp3 player or downloaded songs. Since windows 10 Anniversary Update (recent version) which removed the support for WMDRM tracks. This is the reason for the issue. Napster songs are all WMDRM protected songs.
Some of our customers already reported this issue. Microsoft removed the DRM and we cannot put it back.
We are working on a Windows 10 app that will instead allow download and offline playback.
In the meantime, to resolve this issue, we recommend you not to proceed with this Windows 10 Anniversary update.
If you have already installed the Windows 10 Anniversary Update, then please uninstall it as per the steps are given below:
Select Settings > Update and Security > Recovery and select the Go back option.
You can get the detailed information about the issue from the article given below :
You can also follow the instructions given in this article to uninstall the Windows Anniversary update:
Once you have reverted to windows OS version, you will be able to use your device.