Forum Moderators: open
you have set your css paths reltive to site root folder "/some-folder/mycss.css", while you forgot to create a site with dreamweaver (so as to know whats your root path).
This will cause the conflict you describe, espcially when you place your project/site inside other folder(s).
Go to Site ->New Site and set up your working site properties. Hopefully this will clear things up for you.