Managing "HTTP/1.1 502 Bad gateway error" - http

Question

I need to interact with a remote HTTP server at the lowest possible level (i.e.: at socket level) because my target is a very small embedded system with no support for higher level libraries (it's a bare-metal uController wit no O.S. at all and talking to a GSM modem via serial line; modem has some support for sockets, but nothing above that).
Basic need is to upload a "file" using POST.
I have all needed Header/Body in place and it "usually works".
Problem is I randomly get a "HTTP/1.1 502 Bad gateway error" response and this is more likely to happen as the size of "file" increases.
I understand this means there's some problem between the reverse proxy frontend (nginx, apparently) and the backends, but I have absolutely no control on those (actually I dont't really know how the atual setup besides what can be gleamed from (light) probing).
My current strategy is to open a plain socket and send the folowing sequence (dots represent binary data):
POST /path/to/websend.php HTTP/1.0
Host: host.domain.tld
User-Agent: Mozilla/5.0 (Windows NT 5.1; rv:33.0) Gecko/20100101 Firefox/33.0
Connection: Keep-Alive
Proxy-Connection: Keep-Alive
Content-Type: multipart/form-data; boundary=AaB03x
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
Accept: */*
Content-Length: <full_length>
--AaB03x
Content-Disposition: form-data; name="IV"
Content-Type: application/data
Content-Transfer-Encoding: binary
000102030405060708090A0B0C0D0E0F
--AaB03x
Content-Disposition: form-data; name="S_TXT_FILE"; filename="FILENAME_s.txt"
Content-Type: application/data
Content-Transfer-Encoding: binary
..............................................................
..............................................................
...... several 512byte blocks ................................
..............................................................
..............................................................
--AaB03x--
Is there something I could do to enhance reliability?
I already do multiple retries and this actually works, but sometimes I need to retry six or more times to have a positive answer (200 OK).
Note I send exactly the same sequence on rety and it succeeds... eventually.
I need to send two parts because content is encrypted and first part is the neded "Initialization Vector".