C# COM Exception: 0x80010108 (RPC_E_DISCONNECTED))

For some reason I have started to get the below exception in my code after it has been working perfectly for over 6 months. There have been no code changes at all but here is the main bit of the exception:

System.Runtime.InteropServices.COMException (0x80010108): The object invoked has disconnected from its clients. (Exception from HRESULT: 0x80010108 (RPC_E_DISCONNECTED)) at Microsoft.Office.Interop.Excel.WorkbookClass.Save() at ditCreditEMGTestAutomationDifferenceEngine.Comparers.ExcelComparer.performProcessClean() in 

And the code it seems to be referencing:

    private void performProcessClean()
    {   
        xlWorkBook1.Save();
        xlWorkBook2.Save();
        xlWorkBook1 = null;
        xlWorkBook2 = null;
        xlApp.Quit();
    }

I've tried the dll's on two seperate machines where they have been working fine previously but see the same error.

Any ideas?

Installed any updates? Service Packs?