SharePoint Designer 404 error

Hello,

I have a SharePoint 2010 Farm and suddenly I am unable to open sites in SharePoint Designer. Please note that this has started to occur since last some days only.

Whenever I try to access the sites, I get following error

404 - File or directory not found.
        Server Error   404 - File or directory not found. The resource you are looking for might have been removed, had its name changed, or is temporarily unavailable.

What could be the issue ? Please note I am able to access all the sites when I try to access them through browser. 

Thanks, Tejas