I have Homeassistant running with a reverse proxy setup - originally nginx as an addon in home assistant and then more recently for other purposes cloudflare zero trust.
This allows me to login to Homeassistant when out and about and for my watch to update.
Over the weekend this broke for Homeassistant and because I had made an upgrade to my router thought it was due to that so was scratch my head as to the reason why.
Today I’ve finally sorted it and it is due to a recent update of Homeassistant which had a breaking change to do with reverse proxy.
There is a setting in the configuration.yaml file to authorise the proxy ip.
https://community.home-assistant.io/t/home-assistant-400-bad-request-docker-proxy-solution/322163
This is the article that explains it.