This error is mostly created by empty folders in /data/songs/ folder in your FoF directory or corrupted song.ini files.
If you don't know which song is causing the problem and want a quick fix to make FoF work again, have the game select a different song on startup. To do this, follow these steps:
1. Open fretsonfire.ini
2. Under [Game] find selected_song =
3. Clear the contents of it so that it displays "selected_song = "
Doing the above will temporarily fix your error, but to actually resolve the problem, you're going to have to figure out which song is broken. The following will save you from having to go through every song folder indivdually checking it. Even so, this process might take a while if you have a lot of songs, so be patient.
1. Try moving all songs out of the /data/songs/ folder to another place. Then move first ten folders back into /data/songs and try running FoF.
2. If it runs properly, close it and repeat step 1.
3. When it stops running, check the last ten folders you have moved back if they are not empty. Also, make sure that your song.ini file looks similar to:
[song] count = 1 name = *Trippolette artist = Andrew Buch cassettecolor = #cc44ee scores_bass = 63657265616c310a310a646963740a300a72300a scores_lead = 63657265616c310a310a646963740a300a72300a scores_rhythm = 63657265616c310a310a646963740a300a72300a scores = 63657265616c310a310a646963740a300a72300a
A good habit to develop is checking new songs that you download to make sure that they have all of the information in song.ini that they need to function. The very minimum a song.ini file should include in it is the [song] header, the "artist = " string, and the "name = " string.