I needed to create a second instance of a craft 2.0 instance and so I copied the current instance to a new instance. I followed the directions found here: https://craftcms.com/guides/duplicating-a-craft-site
The structure is like this:
/var/www/html/my_site (original - contains the craft and public folders)
/var/www/html/new_folder/my_site (this contains the craft and public folders)
There is a reverse proxy (Apache) that handles routing of the URL's. The website comes up with no problems. But, I cannot sign into the site to administer the site. All I see is "An unknown error occurred." There is nothing in the craft.log in the instance that indicates what the unknown error is.
Is there a value in the database tables that might be incorrect? I have scanned for references to the original path and I don't find anything. The document root in the Apache config files for the instance is correct. There was a recommendation to delete the runtime folder after copying a Craft install and I have done that but I still get the error and there are no errors in the craft.log file.
One last interesting observation. When I try the admin URL for the copied site e.g. http://copy.mysite.com/admin the login page for the original site is displayed e.g http://orig.mysite.com/admin/login. So, somewhere there is a reference lurking to the old site I copied. I have changed general.php, db.php and search for references to the old host name but found nothing.