Thank you so much for this solution. I was wondering why my desktop app was not playing music for the last couple days - although when I start music from the web player I was able to listen & control it via the desktop app.
After your post I remembered that I have recently changed my sound settings to highest available. Reducing it to "32 bit, 19200Hz" solved the issue. Many thanks!
there is no problem with hosts file, it is empty. Also running Spotify as administrator does not eliminate the problem. On the firewall side one problem may be the resource allocation in firewall itself and another more probable seems the situation that pieces of audio files between Spotify app and server do not move in order. That means something like "when downloading files server responses with HTTP 206 "Partial content" and sends the file in parts. When the first part is not the beginning of the file, there is a difficulty tracking the whole file and therefore for security reason, Security Gateway is blocking this part."
Zorgglub, thank you. Renaming local-files.bnk fixed my problem of all imported local files would not play. I also had to tell Spotify desktop where my local files were, it didn't point to my Music folder anymore. I am a transfer from Google Play so I was missing over 1500 tracks because of this error. Thanks again!