Auto-Fill not working with Windows desktop app

This is the setting I was talking about. Should work with that. Difficult to say why the integration doesn't work for you. As far as I know, not every browser is supported, only a collection of widely known browsers by well known vendors. As far as I know, this is for security sake: the extension only works fully, if it's an official release of an officially supported browser that is also digitally signed by the browser vendor. This is to avoid using custom browsers where the vendor might have changed/compromised the sandbox where extensions are running in, so credentials could leak.

I was always talking about Chrome and Firefox. With these browsers integrating with the desktop app is definitely working as I wrote, since I use them myself. And it might be some security suite/virus scanner suite is blocking communication for you. I use the default Microsoft Defender on Windows 10 and 11, and with this everything is working fine.

By the way, you can also switch the desktop app to German language. It's translation is ok.

Finally, try this:
Change to the desktop app and go to its settings->Browser and make sure the integrating with the browser is activated here as well. Activate if it wasn't and check the browser again.
If it was already activated, uninstall the browser extension, restart the browser, then switch back to the 1Password app and click the big "Get 1Password for your browser" button. This way the newest extension for your browser should be automatically linked and you can install it again. It should pick up the desktop app login and just display your 1Password data without signing in again. At least this is how it is supposed to work in Chrome, and I just did that the other day.