Copy of Craft instance generates "An unknown error occurred" for login 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)

copied to:

/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.
So I am going to answer my own question here: The solution was that something, I don't know what, what messed up in the DB. So I dropped the DB and then copied it over again from the original. That fixed it.  I never got an indication in the logs that the DB was the issue I just decided to start with replacing it and then work backward if necessary. 

Thanks for the tips. At least it eliminated the obvious issues.