If you receive an error message “An unknown error occurred. Please check your library and try again.” when you use an iOS device such as an iPad or an iPhone, it may be because you are using the Remote app to attempt to control your iTunes server and your server has no sound card.

 

The Remote app is used to remote control an iTunes server and play music either on the server's itself (i.e. through its sound card) or through an AirPlay device on your network (such as an Apple TV or dedicated AirPlay speakers).  The Remote app cannot be used to stream music to your iPhone, iPad or iPod Touch.

 

Instead you should enable Home Sharing on your iOS device and use the native Music, iPod or Videos App on your device.  Please see Push model: Playing your media on an AirPlay device for further instructions on how to do this.

 

Please see Two models - push or pull? to better understand the difference between using the Remote App to push music/video to a device vs the built-in apps on your Apple TV, iPhone or iPad to pull music/videos from your server.