[TYPO3-english] Remove extension manually (Typo3 6.2)

[joel zimmerli]

Just after I installed yag_gallery 3.2.7 on Typo3 6.2 almost all the "admin" menus in BE (extension manager, languages, install, etc.) are not available anymore. When I click on the menu, I received the following error:

Oops, an error occurred!
Class Tx_PtExtbase_Utility_FeBeModeDetector does not exist

As I canot use the Extension manager menu, I'm wondering how I can delete(uninstall) this extension?

I already tried to delete the entry "yag" in LocalConfiguration.php and deleted all files under typo3temp but I still get this error.

If I just rename the "yag" directory under typo3conf/ext, the whole BE access is not available (blank screen!).

What is the right way to delete this extension manually?
Thank you in advance for your help.
Regards,
Joel