Thread:Project:Support desk/Update previous MW 1.16.5 to 1.17.0 throws an 500 internal server error/reply (34)

That's the (old) standard skin. If you're getting that it means the vector skin does not exist, and you did something very wrong. The version of vector that ships with 1.16 is not going to work with 1.17. Please just download MediaWiki and use the version of vector that is in the tar file you download (The only thing that you should keep from your older version when upgrading is the extension directory, the image directory, the math directory [if using texvc] and your LocalSettings.php). Upgrading only some directories of a MediaWiki install and not others is just going to screw things up...