Hi
As far as I know this error occurs when PI tries to call ECC using HTTP connection and in the ECC side
on that time no system resource is available to response to that call.
If we start the ICM then this error resolves.
If we restart the message in PI after some time , then also the message processed.
For the permanent fix , I think we need to increase the load in ECC apps server.
Please check with basis team and they will be help you better.