Solution : https://service.sap.com/sap/support/notes/483451 (SAP Service marketplace login required)
Key words : 
sap web application server communicate, sap mapi service provider, sap internet mail gateway, generated rfc trace files, control panel system variables, sap exchange connector, bc-con interface, remote function call, rfc_trace=1 rfc_trace_dump=1 rfc_no_compress=1, called dev_rfc<nummer>
Related Notes : 
       
| 371906 | Clarification of a WEB Reporting problem | 
| 313971 | Monitoring: RFC error messages for SAPSYS users | 
| 308929 | Activating trace for SAP Logon Control | 
| 176277 | Generating RFC trace information | 
| 65325 | Error files and trace files for Remote Function Call (RFC) | 
| 40024 | Transferring customer files to sapserv# using ftp |