General
Follow the instructions in chapter Incoming communication (to Connect).
Try to make the required API call using transaction /IWFND/GW_CLIENT and check the error messages that appear.
General communication error (e.g. HTTP client - communication error)
First, determine the full destination URI of the communication.
When using HTTPS, first make sure that the SSL certificate used by the target server has been stored in the STRUST transaction as a trusted certificate for the "ANONYM" SSL client.
Call transaction SMICM and check the current trace file. This should contain more detailed information about the cause of the error. If necessary, increase the trace level to the value "2" and repeat the communication attempt to obtain more detailed information.
If the SAP installation consists of several application servers, repeat this procedure for each individual application server. Use the remote login function of transaction SM51 for this.