I do a regular sports day pass when i wish to watch something which i normally chromecast to the tv.
This stopped working last week showing error 2100, online help just kept saying i was watching on 2 devices but that wasn't the case, only one device via android phone.
It counts as two devices if you start them in the wrong order.
So start the programme on the Android phone , and only then cast it.
(Or vice versa, if that doesn’t work),