Download not possible

[dermsh]

Plan

Premium

Country

Germany

Device

iPhone 7

Operating System

iOS 12.2

My Question or Issue

We have a family plan and my wife is not able to download "new" albums/audio books. New means: she hasn't downloaded this album before. If she downloaded it before, she is able to downlaod that album for offline listening.

When pressing the download button at such "new" albums, an error appears (translation from German version by myself): 

Completely full
Congratulations, you have a great collection. Unfortunately you don't have a place in the "library" anymore. If you want to save new songs, you first have to delete some old tracks or albums.

What we have tried: emptied the spotify cache, removed the app from the iPhone. When I log in with my account, I can downlaod all albums. When changing back to her account, the error is back. Within the Spotify settings I logged her account out of all devices. I don't know what else to do. 

Any ideas?