On the upper left menu, Scene -> Quick Open Scene... , does that Login.tscn appears?
if not, can you open another scene and drag-and-drop that scene you're having trouble with? For instance: create a scene1.tscn, and drag and drop Login from the resource directory to the screen? If that works, try the "open in editor" button that appears on the node tree.
If you can somehow workaround and open the scene, be sure to right click on its tab and then click Show in the Filesystem. There it should show where the file is.
If none of that works, try copy-pasting the scene with your file explorer (windows explorer, finder, etc) and checking the project again.