We have a very simple single server SharePoint 2013 setup that has been operating fine until I tried to extend a Web Application to an 'Extranet'.
After extending, the original web application's root/default landing page of the only site collection (www.site.com) displayed the "Sorry, this site hasn't been shared with you" for all users (even the admin) HOWEVER,if anyone navigates to www.site.com/lists/mylist/allistems.aspx it displays correctly and you can access the rest of the site no problem.
I've tried updating the Welcome page, updating the home page to something else, completely rebooting the server, but no luck.
Any ideas on what to try next?
BTW, I'm using NTLM.