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The old app (from March 2020) had stopped working, so not updating wouldn't have helped. If you're saying that casting only stopped working this week then that's something different - what error are you seeing @Robert_05 ?
That message specifically means your BT Sport subscription is included as part of your BT Mobile contract, and so doesn't entitle you to use any large screen apps (including Chromecast). That won't have changed any time recently unless you changed your BT Sport subscription.