This error is caused by one of FoF 1.2.512's many bugs in it's coding. The only known way to solve this error is by downgrading to an earlier release, like 1.2.451 or RF-Mod 4.15. This means you are going to have to completelly uninstall your current version. If you have a lot of songs already installed and do not want to delete them, move them out of the "FretsOnFire/data/songs" directory before doing the following:
- Delete your current FretsOnFire directory,
- Delete your fretsonfire.ini, and the fretsonfire text document (Start > Run > %appdata%/fretsonfire/)
- Install one of the above versions, making sure to follow any installation instructions EXACTLY,
- Move your songs to the new version's "songs" directory.