Why do I get RPC_E_SYS_CALL_FAILED when calling from Excel vba a method in a Com Interop dll ?

I have a C# Com Interop dll that contains some small methods. These methods are being called from Excel vba. Example: Call oMethodWrapper.StartExplorer(sExtractionPath, True, True).

At one client of about 280 one receives next error;

It says: System request failed. (Exception of HRESULT : 0x80010100 (RPC_E_SYS_CALL_FAILED)) The problem is not consistent, sometimes it pops up. Do you know what is going wrong here ?