I don't have any custom menu music, this is my first time modding smash, I also noticed that even if I delete all the files from the salty sd folder leaving it blank it still acts the same way (title screen is fine, main menu crashes). turning off saltysd in smash selectors options will make smash work just fine. Maybe I don't have it set up right/isn't compatible with my version? Perhaps I should try deleting my smash update and seeing if it works...It could be a corrupted menu music file.
If you remove the "bgm" folder, or rename it to an unusable folder name such as "bgm1" and boot up smash and have no issues, then your problem is the custom menu music. If it still crashes I got nothing.