Based on the error I'd guess that you have a blank line of code on line 1 of the page: contacto-pronet-lactacyd.php
Php is very particular about not having blank lines of code before starting a session or redirecting.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.