Thread:Project:Support desk/Fatal error: Class 'XXXXXXXX' not found

I have a MediaWiki 1.19.2 installation (http://molwiki.org) with PHP 5.3.18 and MySQL 5.1.63. I've tried to upgrade it to the 1.20.0 version according to the manual. After uploading all the files to the server I pointed the browser to http://molwiki.org/w/mw-config/ and got an empty page with an error string "Fatal error: Class 'ConfirmAccountUISetup' not found in /usr/www/users/unexpr/w/extensions/ConfirmAccount/ConfirmAccount.php on line 42". Then I've switched off this extension in my LocalSettings.php and tried again but it complained about another extension and also in a sense like "Class 'XXXXX' not found in XXXXX". How to solve the problem?