How to fix Magic GarageBand missing files?

The clue that explains how to fix your issue is that the applications works in a brand new user account. If it were a problem where the files were truly missing or a bug in the app - you would expect the same error in a new account. Unfortunately knowing that the problem lies in your library isn't much help due to the large number of files exist in most library folders.

To fix this, several different paths could be taken:

1. You can import the actual files into the new account from the old account and side step pinpointing which file is problematic.
2. You can call the engineers that have the source code (AppleCare in this case) and see if they have a knowledge base explaining which file or files or directory needs to be cleared from your library to avoid doing the work involved for one of the other DIY choices.
3. Do a search of your library files (I would do a binary search - eliminating half of the files each pass - or as close to half as you can manage without software to automate tracking which files were deleted and which were not) and testing to see if the error persists after you have removed one or more files from ~/Library
4. Use a tool like fs_usage to try to track each file the GarageBand app opens in hopes of isolating the ones from ~/Library that it needs to have reset to default.
5. Hope that something like user permissions in the Library folder can be seen to be off and adjusting them by hand (Disk Utility won't repair your home folder permissions or look deep in ~/Library to fix things sadly) or try throwing away simple things in ~/Library like the GarageBand plist files and cache directories.

Again, it's a bit like searching for a needle in the haystack and I realize this isn't the same as telling you "file X and cache directory Y are the culprit" since I don't actually have that level of detail on iLife 11 files.

Hopefully this will help you or someone else in narrowing down how to track corruption in a user account enough to decide to call the vendor or just spend the time you need to move music, photos, documents over to the new account and just hope the error doesn't crop up again.

I'd quit GarageBand app, make a back up of my system and then move these files/folders to the trash:

/Users/whatever/Library/Caches/com.apple.garageband
/Users/whatever/Library/Preferences/com.apple.garageband.LSSharedFileList.plist
/Users/whatever/Library/Preferences/com.apple.garageband.LSSharedFileList.plist.lockfile
/Users/whatever/Library/Preferences/com.apple.garageband.cs
/Users/whatever/Library/Preferences/com.apple.garageband.plist
/Users/whatever/Library/Preferences/com.apple.garageband.plist.lockfile
/Users/whatever/Library/Saved Application State/com.apple.garageband.savedState

If that doesn't remedy the issue, then I'd call Apple if I had AppleCare or could visit a genius bar and then just hand migrate my user files to a new account. The latter should take less than 20 minutes to plan and the files can copy while you are eating dinner. Having a good backup is mandatory for any delete / trial / hand migration just in case you find something you forgot to migrate days or months later.

If you open Activity Monitor, then select GarageBand you can click "Inspect", then "Open Files and Ports". If you open GarageBand, and then try to run Magic GarageBand, it will try to open all the files needed for it. You can do this in both user accounts, and copy and paste the text into this diff checker. Any files that are different can then be copied across from the user account that works to the user account that doesn't work.