When I log in to Spotify, the app show me the message:
A firewall may be blocking Spotify. Please update your firewall to allow Spotify. Additionally you can try changing the currently used proxy settings (Error code: 15).
What can I do?
Well, if you do not have any firewall nor a proxy, then try a restart …
That error seems to appear on their support page for Windows and MacOS as well.
See their Linux support channel Help - Desktop Linux
There was just a Spotify outage:
…Spotify is coming back for users across the web. If you were logged out of your account, try logging back in to access your music collection.