zlib's uncompress() strangely returning Z_BUF_ERROR

I'm writing Qt-based client application. It connects to remote server using QTcpSocket. Before sending any actual data it needs to send login info, which is zlib-compressed json.

As far as I know from server sources, to make everything work I need to send X bytes of compressed data following 4 bytes with length of uncompressed data.

Uncompressing on server-side looks like this:

/* look at first 32 bits of buffer, which contains uncompressed len */
unc_len = le32toh(*((uint32_t *)buf));
if (unc_len > CLI_MAX_MSG)
    return NULL;

/* alloc buffer for uncompressed data */
obj_unc = malloc(unc_len + 1);
if (!obj_unc)
    return NULL;

/* decompress buffer (excluding first 32 bits) */
comp_p = buf + 4;
if (uncompress(obj_unc, &dest_len, comp_p, buflen - 4) != Z_OK)
    goto out;
if (dest_len != unc_len)
    goto out;
memcpy(obj_unc + unc_len, &zero, 1);    /* null terminate */

I'm compressing json using Qt built-in zlib (I've just downloaded headers and placed it in mingw's include folder):

char json[] = "{\"version\":1,\"user\":\"test\"}";
char pass[] = "test";

std::auto_ptr<Bytef> message(new Bytef[             // allocate memory for:
                             sizeof(ubbp_header)    //  + msg header
            开发者_开发百科                 + sizeof(uLongf)       //  + uncompressed data size
                             + strlen(json)         //  + compressed data itself
                             + 64                   //  + reserve (if compressed size > uncompressed size)
                             + SHA256_DIGEST_LENGTH]);//+ SHA256 digest

uLongf unc_len = strlen(json);
uLongf enc_len = strlen(json) + 64;

// header goes first, so server will determine that we want to login
Bytef* pHdr = message.get();

// after that: uncompressed data length and data itself
Bytef* pLen = pHdr + sizeof(ubbp_header);
Bytef* pDat = pLen + sizeof(uLongf);

// hash of compressed message updated with user pass
Bytef* pSha;

if (Z_OK != compress(pLen, &enc_len, (Bytef*)json, unc_len))
{
    qDebug("Compression failed.");
    return false;
}

Complete function code here: http://pastebin.com/hMY2C4n5

Even though server correctly recieves uncompressed length, uncompress() returning Z_BUF_ERROR.

P.S.: I'm actually writing pushpool's client to figure out how it's binary protocol works. I've asked this question on official bitcoin forum, but no luck there. http://forum.bitcoin.org/index.php?topic=24257.0

Turns out it was server-side bug. More details in bitcoin forum thread.