I have encountered the following problem:
Openning joomla administration interface show me, that my Joomla installation is unknown and upgrade, or reinstall joomla files , does noty solve problem.
solution:
Is neccessary to find file "configuration.php" in your root directory on web server and open it in editor and find the line containing
public $live_site = ' something ';
and replace it with empty argument
public $live_site = '';
save the file , and reload administrator webpage, and your problem is solved .
Warning: On the end of line public $live_site = ''; are two apostrophes, not quotation marks.