I encountered this error today, along with a log message in the ULS saying: cannot find site lookup info for request uri
When browsing directly to the site home page or any other page it worked perfectly. Just this page /_layouts/15/start.aspx returned access denied error.
I googled googled, and even binged but could not find a solution.
Until it occurred to me – alternate access mappings!!!
It appears I didn’t set the site URL as an alternate access mapping for this web application.
A quick trip to central administration fixed that:
Client on “Configure alternate access mappings”:
Change to your web application:
Click “Edit Public URLs”
And add your new header in any of the text boxes:
done! now it is working as expected.