Google Chromecast Audio TOSLINK audio drop is fixed with firmware version 2.65 on Yamaha RX series.
 Previous firmware versions had issues with fiber optic serial connection where users encountered audio drops.
 Make sure your Amplifier has firmware version 2.65 or later and your Chromecast Audio device has firmware version 1.40.156414
 
 A bug has been found in Chrome Cast Audio firmware version 1.56.467166. The TOSLINK audio will not function due to Chrome Cast Audio freezing
 after the firmware update (only happens when your device is left on for a long period of time) with casting applications like Spotify and Deezer.
 Factory reset your Chrome Cast Audio by holding in the button on the side for 10 seconds. Wait 45 seconds after the factory reset.
 Your Chrome Cast Audio should be functioning as normal. (You need to reassociate your device with the wireless signal)
 Note: Your Chrome Cast Audio firmware version should remain as 1.56.467166