[TYPO3] Labels in BE missing

[Georg Ringer]

Hello,

I have got quite a strange problem: Many labels in the BE are not 
displayed, e.g. deleteing Cache, Label of the logout-button, 50% of the 
labels in the tree menu and all labels of the search at the bottom of 
each page in BE.

There is simply nothing displayed!
I checked the core-xml files, the template.php but there should be 
everything fine. It happens with every language

where else can I look? What can be the solution?
It is a 4.0.2

many thanks for help!
Georg