Can't create a Premium Music library anymore

[Bazrat]

I wonder if it something to do with this:-

Important information regarding Premium Music libraries in Plex

We’ve been hard at work on some significant improvements to music metadata in Plex, and as part of that effort, we’re reworking some parts of the system that automatically grabs all that sweet artwork and other metadata that make your music library awesome. This change will impact the ability to add new music to Plex Premium Music libraries on older Plex Media Servers: Starting July 1, Plex Media Server version 1.15.8 or higher will be required to continue to add music to Premium music librarie…

Even though you are using a version above 1.15.8. Maybe a bug while they were sorting this out?