I understand the ignore function in the playlist, but the folders section? It's irreversible! Seriously? I bet this is a bug or there is a very well hidden solution for it.
How can I fix this? Without placing the song elsewhere/renaming it...
Edit:
I opened neutronmp.db with SQLite Database Browser and deleted plist_music_ignore_v2, where the path to the song was located, but it was still ignored even after a refresh. In settings I got Service option checked, so I closed Neutron and flushed RAM, started Neutron again, rescanned source manually by long-click -> SCAN, checked the song an it was there. Finally. After 2 hours of searching for a suitable program and the right line to remove, trial and error. To whoever made this the only way to fix it: "Good one! But not funny."

Anyway, problem solved but the bug still remains and needs to be fixed.