Steps to reproduce:
1. Download and open attached project
2. Click on Assets\Export Project menu option. Error will be thrown:
Error while exporting package: Couldn't copy metadata
UnityEditor.AssetDatabase:ExportPackage(String, String, ExportPackageOptions)
ExportProject:ExportThisProject() (at Assets/ExportProject.cs:10)

Code to export package:
AssetDatabase.ExportPackage ("Assets", "MyPackage.unitypackage", ExportPackageOptions.IncludeLibraryAssets | ExportPackageOptions.Recurse);