A method was called at an unexpected time

You need to wait for the async method to complete. So you could use the new await as one option:

var files = await myStorageFolder.GetFilesAsync();

You might want to check the documentation on dealing with async methods here.

If you don't want to use the asynckeyword (in my case, the code was inside a property, so asyncwasn't an option), you can use a TaskAwaiter instead, by chaining these two methods:

var folder = Package.Current.InstalledLocation.GetFolderAsync("folderName").GetAwaiter().GetResult();

This won't throw a InvalidOperationException nor cause a deadlock.