Error 400 When Running UI Flow

I am getting the below error while calling UI Flow from Power Automate.

The UI Flow is as simple as can be. It only opens Notepad, and I've successfully tested it.

The gateway is installed in standard mode on a single desktop computer that has access to the internet. I installed it and configured it using the same account that I'm testing with in Power Automate. I also made sure to install the Flow UI components and browser extensions (enabled in both Chrome and new Edge). I checked the local firewall logs at that time as well, and I'm not seeing anything being blocked.