Why does horizon 4 always kick me out of online gaming mid session?SO FRUSTRATING

I would strongly recommend giving the troubleshooting steps below a try:

Settings > Network Settings > Advanced Settings > Alternate Mac Address > Clear

Then select System > Settings > devices and streaming > blue ray > persistent storage > clear three times > restart console.

Also, It would be great if you can delete the local save of the game.

If the issue persists, please consider taking a look at this article: Troubleshoot your network connection speed | Xbox Support

If by any chance you are still encountering the same issues, I would recommend reaching out to the game developers for further assistance!

Are you having any connection issues with other games as well, or is it specifically on Forza Horizon 4 only?

If you try a Horizon Solo session, and then try to switch online, does it give you the same error message?

Same. Kicked me out mid-game and now i cant play anything that requires me to be online. Hope this is resolved soon

So I talked with the support of xbox and they told me easily that for Forza Horizon 4 the Xbox Live is mandatory for playing online. Bought it, activated it and I could immediately join the online session.

Sometimes lagging out of game sessions looks like a "Kick From Session". It happened to me a couple of times and my friends more so. No warnings were given for me, though.