Keep in mind though that it is in early beta state. So it is fairly stable but missing a lot of features. Also I have tested it only on a very limited amount of devices.
The login issue could have something to do with the webview itself. Can you try to reset the app cache and try again to login? If it does not work please open a issue on Github.
Regarding the empty view: https://vauvenal5.github.io/yaga/quickstart/
Currently you still have to change the path in the settings after login. Yaga does not automatically scan your cloud for images.
I’m a bit confused by how this app works. It seems to have its own local data directory, rather than just sharing the local data directory used by the main Nextcloud app. This means every file needs to be downloaded twice. I tried changing Yaga’s local data path to match the Nextcloud app’s data path but then everything stopped working (no files showed up, loading icon never disappeared).