I also have Windows 11, I went to the Diablo 4 folder and set the Diablo IV and Diablo IV Launcher files to run as administrator.
Then I start the Diablo IV Launcher file, which starts the B-Net Launcher as Administrator and when I hit the start button for Diablo IV it asks, if I want to run it as administrator.
This is the only way I can start the game and play it. When I start the B-Net Launcher via it’s usual shortcut, I still get the error 1909.
Hope this helps everyone else with the issue, until they solve this.