It seem PLC S7-1200 use form authentication, which require preserve cookie between login url and protection url. The arduino by alone is hard to preserve cookie if not impossible.
HTTP/1.1 200 OKDate: Fri, 28 Apr 2006 03:14:58 GMTServer: Apache/2.0.46 (Unix) PHP/4.3.2Set-Cookie: foo=barSet-Cookie: food=applesConnection: closeContent-Type: text/html; charset=iso-8859-1
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