Tried this, too, but no luck 
I’m having exactly the same issue, with rclone 1.72.0 in Arch Linux. I’ve been trying the solutions above, none worked so far. I’ll keep trying new possible solutions, or another rclone version from AUR, something different, but I guess it’s Apple blocking the access.
same 400 error with user-agent supplied here on Linux Mint 22.2 with rclone 1.72.1
Great shame as I’d tried snap install of icloud-for-linux only to find it is just a web browser so useless for my use case of syncing my iCloud drive to local folder.