If a video was recently uploaded, the server might still be transcoding it. S1026 can appear if the file isn't ready for playback yet.

If the issue is server-side (like maintenance), there is nothing to do but wait for the developers to restore the service. If you are a webmaster/uploader :

In some cases, if a file has been flagged for a TOS violation or deleted by the uploader, the player might trigger this status code instead of a standard "404 Not Found."