There is a related post:
Windows 11 2024H2 - TrayIcon won’t load when starting Duplicati as a service
The “Open” command tries to first create a sign-in token, which is a special short-lived token that can be used instead of the password. If the sign-in token is generated, it will open the /signin.html page with the token, and that will cause the browser to obtain a valid login.
If that fails, it will just open /index.html and let the user sign in.
The hang that I previously fixed was caused by a deadlock on the signin page, where the TrayIcon UI thread would be blocked by the call, and the call wanted to return to the UI thread with the result. (For developers: missing ConfigureAwait(false) ).
I am guessing that the issue described here is something similar.