When the designer (or anyone else) tried to modify an aspx page or master page of one of our public sites, he could open aspx pages, but when he tried to save those or when he tried to open a master page, he got the following error:
"The server could not complete your request. Contact your Internet service provider or Web server administrator to make sure that the server has the FrontPage Server Extensions or SharePoint Services installed."
Clicking on "Details" gave a blank error. A lot of googling suggested recycling the application pool (didn't work) or extending the web application (that didn't apply to our situation, since that was when you were using FBA in stead of Windows authentication and we were using Windows authentication). The last option made me take a look at the security settings though, and that's where I found a solution that worked for me. It turned out that "Client Integration" had been turned off.
To enable this (and hopefully resolve your problem), go to your Central Administration and then navigate to Application Management --> Authentication Providers --> Default. On this page, you can enable client integration. For me, that did the trick...