"Query failed" points out that there is an error in the usage of the mySQL database.
To get rid of this error we have to take a look what is causing this error message. For this you have to activate the debug mode to see the real error message. Open the _settings.php file in a texteditor and look for the following line...
...and replace the OFF with a ON
Reload the page where you did get the error and now you'll see a more detailled error message.
If you don't have a clue what this new error message means just contact us via the forum or the irc chat and ask politely for help and provide this error messages for us.