R – Browsing a SharePoint publishing page returns a 404 error message

I've changed the master page on a publish site collection. When I browsed back to the start site to check the new master page the browser just returned a 404 error, telling me that the page could not be found.

But when I open the site collection with the SharePoint Designer every thing seems to be ok and in its place. All pages are as expected in the page library. Within the Designer I could even open the pages for editing.

So does anyone has an idea why the browser cannot find and open the pages?

UPDTAE

To clarify my problem I should say, that also switching back to the default master page doesn't bring back the site. I still get a 404 response. So the problem shouldn't be directly related to my custom master page.