Ok. Since my original post on this thread (13/09/2020), several people have suggested the problem is caused by someone on the same network having Google Chrome open. So, last night I waited for everyone to go to bed and all computers to be switched off. I then booted by tablet, opened the Now TV app and attempted to cast one of the trailers. Voila! It worked! No problem at all. Also, I managed to stop the cast without issue as well (more of which later).
I tried again this morning before any computer had been switched on. Again, the cast worked fine. I then booted my laptop and opened Google Chrome, tried to cast from my tablet and got the error message! All that happens is an advertisement (a picture, not a video) is cast to the TV (currently I Hate Suzie).
Furthermore, when I try and stop the cast (of the picture), this too fails. It shows on my tablet as having stopped casting, but the picture remains on my TV. If I close the Now TV app and reopen it, it shows as still casting! The only way to stop the cast of the picture is to open the Google Home app and stop the cast from there.
It doesn't matter if the device is using WiFi or Ethernet to connect to the internet, as soon as someone opens Google Chrome it prevents any attempt at casting Now TV.
The only things I haven't tried yet are: (1) casting from Now TV and then opening Google Chrome; and (2) attempting to cast when another browser (e.g. Firefox) is open. My guess is neither will cause Now TV to stop casting. However, opening Google Chrome after successfully casting will probably prevent me stopping the cast without going into the Google Home app.
I have today fed all this back to Sky/Now TV by email to [email removed] and would urge others to do the same. Clearly, this is a major issue and the more people that complain the more chance Sky will do something about it!
Apologies for the length of this post, but I hope it provides some help to other frustrated users having the same issue. If anyone comes up with a permanent fix, please post it asap. Thanks.
Thanks. It looks like the issue, I just tried it, and when having chrome open on my laptop, now TV does not cast (or stop casting as described). Tried it on my desktop, and it didn't seem to to have the same effect, not sure why as they are all on the same network!
I just stumbled across your post, so first and foremost thank you for the work around.
I don't know if you did any more testing on this but I just started something casting and then started Chrome about a minute into the cast, to which the cast continued.
I was then able to stop and disconnect the cast from within the app, without having to close Chrome.
Then, while Chrome was still open, I was then able to re-connect to the Chromecast from the app and then start streaming something to the device. Kind of didn't expect the latter behaviour, but it was almost like with Chrome open, it recognised the device and started working without issue. I also then tried on a second Chromecast device which also didn't work previously and that also began to work without issue.
Something very weird indeed is going on here, but for now, touch wood, everything is working correctly in my house, even when I'm using the Chrome browser.