ST is running well on my Android 9.0 smartphone. I can open the web interface within the Synthing app without any problem.
If I try to open 127.0.0.1:8384 locally via the Chrome browser, a login dialog appears, that asks for credentials to access the page.
I have not set up any username / password. I even can´t find any settings for that in the Android client.
I believe this is a security measure, to prevent other apps on the phone from interacting with syncthing without authorization. The inbuild webview in the app is configured to authenticate automatically.
The username/password used is fixed (username is syncthing, password is the API key).
If you open the web UI from the app, you can go to the settings and see the API key there. Note it , so you can login via Chrome/Firefox/
… browser using “syncthing” as username and the api key as the password.
(the fork has a settings label “tap to copy api key” under Settings > Syncthing options on the Android UI)