Same error. First thing to mention is that I don't have the SD card. All my music is on the internal hard drive.
Tried to re-scan, as suggested. As a result, my music folder and its subfolders are no longer seen by the app.
I go to Settings->Library->Music Folders and select internal storage. Re-scan doesn't find anything.
I go back and tap on + in the lower left corner of the Folder Selection dialog. In response the app tells me to add Storage/Provider Plugin. I have no idea what it means but tap on Continue anyway. On the next screen I see a bunch of folders, one of them is Music. I tap on it and I see all my subfolders. I tap on USE THIS FOLDER blue bar at the bottom and then allow Poweramp to access files in Music.
Now I click on Full Rescan and it finds nothing. Folders: 0, Playlists: 0, Songs: 0.
What am I doing wrong?