I ran into an issue where I was unable to open some files in the beta of SharePoint Designer 2010 from the server (i.e. .css files or .aspx files). I was getting the message:
“The server extensions were unable to access the file “”. Please check the file permissions. Contact your Internet service provider or Web server administrator for more information.
Similarly to the old SharePoint Designer 2007 solution, I was able to resolve this issue by clearing the WebsiteCache directory on my local environment. Just delete all the content from this directory:
Vista/Window 7/Windows 2008:
Refer to this post for more information .