Earlier tonight I received an error message in iTunes 18.104.22.168 (Windows) when trying to download a purchased song. The error message said “An unknown error has occurred (-42018). Please check your network connection.” Well the network was fine so I Googled for an answer and, oddly enough, found no salient results. I went to the iTunes support site and there found numerous posts about the error. The resolution turned out to be simple enough: deauthorize and reauthorize your computer. In Windows, you click under the store menu and select the “Deauthorize Computer …” option. After you enter your password, your computer is deauthorized. Next you just attempt to play one of your previously purchased songs. It will prompt you for your password again. Once you have reauthorized your computer, go to your pending downloads and restart the purchased song. At least it worked for me!