“However long it takes to fix plus however much time we have to spend answering this sort of question”.
I have the same issue here. Android user casting to Chromecast. Have Gen2, Gen3 and Ultra Chromecasts, none of which will cast. I'm unaware of any recent Chromecast firmware updates. None of my devices have automatically picked up any firmware recently. Reinstalled the app several times (last update 26th June 2020 as per Play Store). Rebooted and factory reset Chromecasts. Rebooted modem and router. Even tried jumping on the beta programme to try a newer firmware revision. No luck.
To me, this all points to a platform issue, so as @RoyB said, all we can do is wait.
I'm looking at alternatives at the moment as huddling around my phone to watch the F1 this afternoon isn't an option.