Alternatives: Spotify Web Player loads then goes to a black page
Spotify Web Player black screen. Spotify web player black page.
I think this situation is caused if you attempt to use the Web Player from more than one device, or several times without using the app. The solution appears to be simply to open the Spotify app on your phone or PC, play one song, then try the web player again.
Found this solution at https://community.spotify.com/t5/forums/v3_1/forumtopicpage/board-id/desktop_windows/message-id/59221