I could only find a older locked thread regarding this error, but I have seemed to have found a workaround for this error. I have been getting an error "Spotify login error Cannot connect to Spotify (8)" when using Djay Pro app with my iPhone on T-Mobile network. The previous thread seemed to narrow down the error having something to do with only the T-Mobile network and its ability to connect to Spotify through 3rd party apps like Djay Pro, Serato Pyro etc. The workaround I have found is using a VPN on your iPhone on T-Mobiles network to get past this error. When using a VPN each app worked correctly without error. Just wanted to post my findings and hope this helps someone.