[TYPO3-dev] TYPO3 Fatal Error: Extension key "backend" is NOT loaded!

[Franz Holzinger]

Hello,

I have a website with the latest TYPO3 6.2 master.
Suddenly the backend and install tool have gone and only show an empty 
white page.

The error log says:

[29-Jan-2014 14:33:14 UTC] PHP Fatal error:  Uncaught exception 
'BadFunctionCallException' with message 'TYPO3 Fatal Error: Extension 
key "backend" is NOT loaded!' in 
/home/mywebsite/public_html/mywebsite/typo3_src-master/typo3/sysext/core/Classes/Utility/ExtensionManagementUtility.php:104
Stack trace:
#0 
/home/userpath/public_html/mywebsite/typo3_src-master/typo3/sysext/core/Classes/Core/Bootstrap.php(471): 
TYPO3\CMS\Core\Utility\ExtensionManagementUtility::extPath('backend')
#1 
/home/userpath/public_html/mywebsite/typo3_src-master/typo3/sysext/core/Classes/Core/Bootstrap.php(240): 
TYPO3\CMS\Core\Core\Bootstrap->registerExtDirectComponents()
#2 
/home/userpath/public_html/mywebsite/typo3_src-master/typo3/init.php(61): TYPO3\CMS\Core\Core\Bootstrap->loadConfigurationAndInitialize()
#3 
/home/userpath/public_html/mywebsite/typo3_src-master/typo3/index.php(34): 
require('/home/userpath/...')
#4 {main}
   thrown in 
/home/userpath/public_html/mywebsite/typo3_src-master/typo3/sysext/core/Classes/Utility/ExtensionManagementUtility.php 
on line 104


How can I fix this?

Regards,

Franz