[TYPO3-debian] Error after upgrade to Typo3 4.2.8

[Raf]

Hi,

I upgraded from 4.2.6 to 4.2.8 and have some errors in the backend: When I 
try to login I have: Cannot find tslib/. Please set path by defining 
$configured_tslib_path in index.php.

And if I set the path manually I have an other error telling me that the cms 
extention is not loaded!
That's very strange.

Somebody have that kind of errors?

Thanks a lot!
Raf