[SOLVED] SAPNWRFC connection refused with no error description

I have properly linked libs in my CMake:

set(SAPNWRFC_INCLUDE_DIR "${PROJECT_SOURCE_DIR}/nwrfcsdk/include")
set(SAPNWRFC_LIB_DIR "${PROJECT_SOURCE_DIR}/nwrfcsdk/lib")
set(SAPNWRFC_LIB_FILES sapnwrfc.lib libsapucum.lib)

target_link_directories(appsap_rtf PRIVATE ${SAPNWRFC_LIB_DIR})

target_link_libraries(appsap_rtf
    PRIVATE Qt6::Quick
    PRIVATE ${SAPNWRFC_LIB_FILES})

qt_add_executable(appsap_rtf companyClient.cpp)

and then I tried a very basic example from SAP NetWeaver RFC SDK:

companyClient.cpp:

#include <stdlib.h>
#include <stdio.h>
#include "nwrfcsdk/include/sapnwrfc.h"

void errorHandling(RFC_RC rc, SAP_UC* description, RFC_ERROR_INFO* errorInfo, RFC_CONNECTION_HANDLE connection){
    printfU(cU("%s: %d\n"), description, rc);
    printfU(cU("%s: %s\n"), errorInfo->key, errorInfo->message);
    // It's better to close the TCP/IP connection cleanly, than to just let the
    // backend get a "Connection reset by peer" error...
    if (connection != NULL) RfcCloseConnection(connection, errorInfo);

    exit(1);
}

//usage: companyClient <hostname> <system Number> <client> <user> <language> <password>
int mainU(int argc, SAP_UC** argv){
    RFC_RC rc = RFC_OK;
    RFC_CONNECTION_PARAMETER loginParams[6];
    RFC_ERROR_INFO errorInfo;
    RFC_CONNECTION_HANDLE connection;
    RFC_FUNCTION_DESC_HANDLE bapiCompanyDesc;
    RFC_FUNCTION_HANDLE bapiCompany;
    RFC_STRUCTURE_HANDLE returnStructure;
    SAP_UC message[221] = iU("");
    RFC_BYTE buffer[1105];
    unsigned utf8Len = 1105, resultLen;
    FILE* outFile;

    loginParams[0].name = cU("ashost"); loginParams[0].value = cU("****"); // hostname
    loginParams[1].name = cU("sysnr");  loginParams[1].value = cU("**");   // system number
    loginParams[2].name = cU("client"); loginParams[2].value = cU("***");  // client number
    loginParams[3].name = cU("user");   loginParams[3].value = cU("*****"); // username
    loginParams[4].name = cU("passwd"); loginParams[4].value = cU("*****");  // password
    loginParams[5].name = cU("lang");   loginParams[5].value = cU("EN");

    connection = RfcOpenConnection(loginParams, 6, &errorInfo);
    if (connection == NULL) errorHandling(rc, cU("Error during logon"), &errorInfo, NULL);

    bapiCompanyDesc = RfcGetFunctionDesc(connection, cU("BAPI_COMPANY_GETDETAIL"), &errorInfo);
    if (bapiCompanyDesc == NULL) errorHandling(rc, cU("Error during metadata lookup"), &errorInfo, connection);

    bapiCompany = RfcCreateFunction(bapiCompanyDesc, &errorInfo);

    // Use a company ID that does not exit. 000007 should not exist in most systems.
    RfcSetChars(bapiCompany, cU("COMPANYID"), cU("000007"), 6, &errorInfo);

    rc = RfcInvoke(connection, bapiCompany, &errorInfo);
    if (rc != RFC_OK) errorHandling(rc, cU("Error calling BAPI_COMPANY_GETDETAIL"), &errorInfo, connection);

    RfcGetStructure(bapiCompany, cU("RETURN"), &returnStructure, &errorInfo);
    RfcGetString(returnStructure, cU("MESSAGE"), message, 221, &resultLen, &errorInfo);
    RfcDestroyFunction(bapiCompany, &errorInfo);
    RfcCloseConnection(connection, NULL);

    // On Windows you can use the following function from windows.h toconvert to UTF-8:
    // utf8Len = WideCharToMultiByte(CP_UTF8, 0, message, strlenU(message), buffer, 1105, NULL, NULL);
    // It will have a slightly better performance than RfcSAPUCToUTF8().
    
    // On 'system i' with Japanese double byte characters using the ASCII LIBSAPNRFC the
    // message string does not contain UNICODE but JIS encoded data. The call to the
    // following function does not make any sense in this case. Instead you can add an
    // encoding to the header lilke this: "<?xml version=\"1.0\" encoding=\"Shift-JIS\"?>\n<message>",
    // and fputs the message directly some lines below:
    // fwrite(buffer, 1, utf8Len, outFile); ==> fputs((char*)message, outFile);
    /*CCQ_SECURE_LIB_OK*/
    RfcSAPUCToUTF8(message,  strlenU(message), buffer, &utf8Len,  &resultLen, &errorInfo);

    outFile = fopenU(cU("message.xml"), cU("w"));
    if (!outFile) return 1;

    /*SAPUNICODEOK_STRINGCONST*/ /*SAPUNICODEOK_LIBFCT*/
    fputs("<?xml version=\"1.0\"?>\n<message>", outFile);
    /*SAPUNICODEOK_LIBFCT*/
    fwrite(buffer, 1, utf8Len, outFile); // See comment above for system i
    /*SAPUNICODEOK_STRINGCONST*/ /*SAPUNICODEOK_LIBFCT*/
    fputs("</message>", outFile);
    fclose(outFile);

    // Now view the message.xml file in a standard browser that supports UTF-8, and you should
    // be able to view the Japanese characters.

    return 0;
}

and the app always debugs only:

Error during logon: 0
R:

I'm 100% of the login data as I use them from other apps from same device (i.e. connecting Excel VBA to the SAP RFC directly without using this SDK connector).

The only difference is that other apps require the loginParameter also system (in the SAP SDK documentation it's as sysid or r3name or dest; I tried all of them, but same result).

How to find out why my connection is being refused with no error description?