This article explains one issue with the Chromecast connection when Intra-BSS Traffic Blocking is active.
Intra-BSS Traffic Blocking
Intra-BSS Traffic Blocking makes sure that the wireless clients cannot talk to each other and is an important part of the Layer 2 isolation.
When this is enabled, the Chromecast won't be able to talk to the mobile phone, laptop or any other clients in the network which will prevent the connection to be established.
How to deactivate it on Nebula
1.) Make sure the Guest Network is off (Layer 2 isolation)
2.) Go into Access Point -> Authentication and disable the Intra-BSS Traffic Blocking
How to deactivate it on the USG/ATP/USG Flex
1.) Make sure Intra-BSS Traffic Blocking is disabled under
Configuration -> Object -> AP Profile -> SSID -> SSID Profile
2.) Make sure you don't have Layer 2 isolation enabled under
Configuration -> Network -> Layer 2 isolation