Facebook Debugger returns 404 but curl works
You should check the server logs if you have a hit like
facebookexternalhit/1.1 (+http://www.facebook.com/externalhit_uatext.php)Its likely that your .htaccess is blocking the Facebook crawler.
the default vhost is used, if the hostname of the machine is equivalent to the request hostname. You can set ServerName somethig-different-from-hostname in the default vhost or disable the default vhost with a2dissite default to prevent this
Turned out it was a DNS problem after all. We changed the primary and secondary DNS from our own to the hosters and after one day it worked.