General
Observe Follow the notes instructions in chapter Eingehende Kommunikation Incoming communication (zu 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 complete 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 STRUST as a trusted certificate for the SSL client "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. To do this, use Use the remote login function in transaction SM51of transaction SM51 for this.
If the error SSSL_SERVER_CERT_MISMATCH occurs, it may help to set the SAP profile parameter icm/HTTPS/client_sni_enabled to TRUE. This can be done temporarily via transaction RZ11 to test if it solves the problem.