Error while calling http_post request

Hello,

My company needs a program to call web services from RPG programs; that's
a really old issue I know, but I have just started to work here so I
started from scratch setting up two programs to test two different methods:
IBM HTTPCLOBVERBOSE UDF and HTTP_POST by SK HTTPAPI lib.

I made the first method working after a bit struggling, but I can't have
the HTTP_POST nor HTTP_POST_XML working at all. I always get error "Only
the http protocol is available" and I can't figure out why.

That's the code:

D url s 1024a varying
D rc s 10i 0
D requestMsg s sqltype(CLOB: 2000000) inz
D resMsg s sqltype(CLOB: 2000000) inz

url = 'https://labelservice.gls-italy.com'
+ '/ilswebservice.asmx/AddParcel'
;
requestMsg_DATA = 'XMLInfoParcel=' + xmlData;
requestMsg_LEN = %len(%trim(requestMsg_DATA));
rc = http_post( url
: %addr(requestMsg_DATA)
: requestMsg_LEN
// : %addr(resMsg)
: '/home/stoppa/testHTTP/testHTTP_result.txt'
: HTTP_TIMEOUT
: HTTP_USERAGENT
: 'application/x-www-form-urlencoded' )
;

where xmlData is some XML code.

I did many tries combining where to put the result data, but I cannot find
anything.
Also, I couldn't find this error message in LIBHTTP source files so I can't
understand it.

Does this sound some bells to you?
Thank you