I’m having this exact issue on the final release of 27. I wasn’t using live activities before I started playing with them tonight, so I don’t know if it’s truly a problem with iOS 27, but it seems like it might be since you mentioned you were on the beta too.
I resolved this issue by toggling Live Activities permission off and then back on in iOS Settings. It seems this forced the app to request and register a fresh token, as the previous push-to-start token had silently gone stale on Apple’s side. I didn’t need to change my automations, scripts, or configuration.yaml.
Thanks for this, I had the same issue. Toggled live activities under iPhone settings/Apps/Home Assistant/Live Activities/Allow Live Activities and this resolved the issue for me as well