gSOAP with OpenSSl (unable to get local issuer certificate) on Windows

Question

I want to work with PayPal api with gsoap in C++. I got my example working on linux, but in windows, I got from 'paypalProxy.GetBalance' "unable to get local issuer certificate".

I already know, that this is a problem, that openssl does not know the trusted CAs. When I issue:

openssl s_client -connect api-3t.sandbox.paypal.com:443 -showcerts | egrep -wi "G5|return"

in linux, I get 'Verify return code: 0 (ok)'. On windows I get 'unable to get local issuer certificate'.

Adding the option '-CAfile .\cabundle.crt' in windows, I also get 'Verify return code: 0 (ok)', where cabundle.crt the mozilla certificate bundle feom here is.

So I wonder, where I have to tell openssl the location of the crt file with gsoap (or even better, systemwide, because the problem does not seem to be in the code)

int main(int argc, char* argv[])
{
    std::string strSignature;
    PayPalAPISoapBindingProxy paypalProxy("https://api-3t.sandbox.paypal.com/2.0/");
    struct soap *soap = soap_new(); // create a new context
    soap_init(soap);

    paypalProxy.header = new SOAP_ENV__Header;
    paypalProxy.header->ns1__RequesterCredentials = new ns3__CustomSecurityHeaderType;
    paypalProxy.header->ns1__RequesterCredentials->eBayAuthToken = NULL;
    paypalProxy.header->ns1__RequesterCredentials->Credentials = new ns3__UserIdPasswordType;
    paypalProxy.header->ns1__RequesterCredentials->Credentials->AppId = NULL;
    paypalProxy.header->ns1__RequesterCredentials->Credentials->DevId = NULL;
    paypalProxy.header->ns1__RequesterCredentials->Credentials->AuthCert = NULL;

    paypalProxy.header->ns1__RequesterCredentials->Credentials->Username = soap_strdup(soap, "XXXXXX");
    paypalProxy.header->ns1__RequesterCredentials->Credentials->Password = soap_strdup(soap, "XXXXXX");
    strSignature = soap_strdup(soap, "XXXXXX"); // Sandbox env
    paypalProxy.header->ns1__RequesterCredentials->Credentials->Signature = &strSignature;

    _ns1__GetBalanceReq ns1__GetBalanceReq;
    ns1__GetBalanceResponseType ns1__GetBalanceResponse;

    ns1__GetBalanceReq.GetBalanceRequest = new ns1__GetBalanceRequestType; 
    ns1__GetBalanceReq.GetBalanceRequest->DetailLevel.push_back(ns3__DetailLevelCodeType__ReturnAll);

    soap_ssl_init();

    if (soap_ssl_client_context(soap, SOAP_SSL_DEFAULT, NULL, NULL, "cacert.crt", NULL, NULL))
    {
        soap_print_fault(soap, stderr);
        exit(1);
    }
    char buf[1024];
    int res = paypalProxy.GetBalance(&ns1__GetBalanceReq, ns1__GetBalanceResponse);
    if (res != SOAP_OK)
    {
        soap_sprint_fault(soap, buf, 1024);
    }
    else
    {
        std::ostringstream oss;
        oss << ns1__GetBalanceResponse.Balance << std::endl;
        std::cout << oss.str() << std::endl;
        oss << ns1__GetBalanceResponse.BalanceTimeStamp;
        std::cout << oss.str() << std::endl;
        //std::cout << ns1__GetBalanceResponse.BalanceHoldings. ->soap_serialize << std::endl;
        //std::cout << ns1__GetBalanceResponse.BalanceTimeStamp soap_serialize << std::endl;
    }