Office automation (Interop) on Windows Server 2012

I'm successfully using Office automation on Windows Server 2008 R2 with Office 2007 in order to convert Office documents to PDFs.
The code is rather simple:

public class WordConvert
{
    /// <summary>
    /// Converts a word file to PDF
    /// </summary>
    /// <param name="sourceFilePath">The path of the word file to convert</param>
    /// <param name="targetFilePath">The path of the PDF output file</param>
    public static void ConvertWord(string sourceFilePath, string targetFilePath)
    {
        object objTragetFileName = targetFilePath;
        Word.Application wordDocument = new Word.Application();
        try
        {
            OpenWord(sourceFilePath, wordDocument);
            SaveAsPDF(ref objTragetFileName, wordDocument);
        }
        finally
        {
            CloseWord(wordDocument);
        }
    }

    private static void OpenWord(object sourceFileName, Word.Application wordDocument)
    {
        wordDocument.Documents.Open(ref sourceFileName);
    }

    private static void SaveAsPDF(ref object targetFileName, Word.Application wordDocument)
    {
        object format = Word.WdSaveFormat.wdFormatPDF;
        wordDocument.ActiveDocument.SaveAs(ref targetFileName, ref format);
    }

    private static void CloseWord(Word.Application wordDocument)
    {
        if (wordDocument != null)
        {
            GC.Collect();
            GC.WaitForPendingFinalizers();

            // 2nd time to be safe
            GC.Collect();
            GC.WaitForPendingFinalizers();

            Word.Documents documents = wordDocument.Documents;
            documents.Close();
            Marshal.ReleaseComObject(documents);
            documents = null;


            Word.Application application = wordDocument.Application;
            application.Quit();
            Marshal.ReleaseComObject(application);
            application = null;
        }
    }
}

The problem is that this code doesn't work on Windows Server 2012.
The error received is:

System.Runtime.InteropServices.COMException (0x800706BA): The RPC server is unavailable. (Exception from HRESULT: 0x800706BA)

Running the code as interactive user (console app) works fine but it fails when running from IIS web app or from windows service (even with 'alow service to interact with desktop').
The user running the app has enough permissions (administrator) and the code works fine with Office 2010.

Any ideas?