This error is caused by having incomplete or corrupt values in your song.ini files. To solve this problem, look in your song.ini files for your songs (This might take a while depending on the size of your song library), starting with the most recent songs you downloaded and going back from there. If you do not know what is acceptable in a song.ini file, the following is a list of safe values:
name = Song Title1
artist = Song Artist1
cassettecolor = Color of the cassette/CD in-game
count = How many times you've played this song (RF-Mod)
frets = Name of fretter of song
scores = Jumbled letters/numbers2
scores_bass = Jumbled letters/numbers2
scores_rhythm = Jumbled letters/numbers2
scores_lead = Jumbled letters/numbers2
1 Required value
2 FoF encrypts these fields to prevent players from typing in their own scores for songs. These jumbled characters are almost definately not your problem, and deleting the strings will delete all your scores for that song.
If there are any other values than the ones listed above in a song's song.ini file, or the file is missing a "name =" or artist =" line, add/remove the string to remedy your error.