[TYPO3-english] problem after upgrading to of 7.6

[Steffen Liebig]

Dear List,

I recently installed typo3 7.5 via our provider's 1-click-routine and 
got it running. So I installed the extensions / copied our webpage from 
6.2 to 7.5. Today, I noticed, 7.6 was available and upgraded the core. 
Then I ran another extension and got an empty page in backend.
I deleted the extension from localconf.php and typo3conf which didn't 
help. So I changed the displayError-setup and got I got the following:

Fatal error: Class 't3lib_extMgm' not found in 
/home/www/typo3751/typo3conf/ext/ext_name/ext_localconf.php on line 5

I found that t3lib_extMgM:: should be changed to 
\TYPO3\CMS\Core\Utility\GeneralManagementUtility::

This lead to the same error message saying that this class can't be 
found either. I don't wonder 'cause I can't find the whole path.

I used to take the t3lib form 4.x for 6.x and it worked, but now I'd 
like to finally get this done. Has it been changed AGAIN in 7.x an if 
so, what has to be done to actualize it ?

Thx for every hint


Steffen