Figure out why uploading to Web servers fail sometimes with `SSLError` error

Trying relay_bw with uploads, with 16MiB instead of 1.5MiB initial size, we sometimes get this error:

relay_bw.py:237, measure: SOCKSHTTPSConnectionPool(host='host', port=443): Max retries exceeded with url: /uploadpath (Caused by SSLError(SSLEOFError(8, 'EOF occurred in violation of protocol (_ssl.c:2393)')))