Due to an unknown reason, my Google account was disconnected from the Google Drive desktop app and I had to re-sign in. Since having done that, my Google Drive folder (the one that was designated as the streaming location) became disconnected from Google Drive and the streaming location has been changed to a virtual drive letter instead. when I attempted to change the streaming location back to the old folder in the app's settings, it failed because "The selected folder must be empty", and that folder is obviously not empty, since it used to be the streaming location until not so long ago.
I noticed that the icon of my old Google Drive folder indicates that it is known by Google Drive, but not synced:

How do I configure Google Drive to use my old folder as streaming location?