I have a server and two clients.
Each client sends and receives SOAP messages to the server.
The number of times sending and receiving messages is hundres or thousands.

When only one client access the server, everything is OK. But when the two
clients sends the message simultaniously, the server *sometimes* returns the
following error message:
ERROR - WSDLOperation:Instantiating the dispatch object for method xxx failed
When this happens, the server never responds any request, so IIS has to be
restarted. Only the SOAP service fails.

I checked the source code (VC++) and everything would be OK. Suppose the
source code would be fine, what would cause the problem?
What should I check?

Thank you in advance.