C++ COM Client calls C++ COM Server exception :The RPC server is unavailable

Here is C++ COM Client application to call Local C++ Com server (it is a use Standard Windows Libraries Service exe) on Win10, It will throw following exception: The RPC server is unavailable (0x800706BA)

I have two ActiveX controls, both of which call the same COM interface of the C++ COM server. After one of the ActiveX controls calls the COM interface, the COM interface can normally send messages to the COM server. However the other one will cause the exception: The RPC server is unavailable (0x800706BA).

Recovery: I disconnect the Server machine's network and reconnect it. Then both ActiveX controls work well.

I have checked RPC options and regedit key, all are right.

What causes "The RPC server is unavailable" exception?