Magento translation problem after moved to another server

Question

After I moved the magento shop (1.8.1) to another server, the language in the frontend is mixed up.

Locale is set to german, but there are a lot of strings displayed in english. The file permissions are currently set to 777.
Reinstallation of the german languagepack didn't help.

A new magento installation on the same server for testing purpose did not show this behaviour: all strings are correctly translated.

After playing around with the files in app/locale/de_DE/, I figured out that the translation is working as long as the string to translate is present in 1 file only. As soon as another file comes up with a translation for the exact same string, the translation is falling back to english.

For example I have deleted all files except Mage_Catalog.csv, which contains the desired string to translate => the translation is working. Now I add for example Mage_Checkout.csv, which contains the exact same string => translation isn't working I can add as many .csv files as I want, as long as the specific string is unique in all files, the string is translated correctly.

Nothing is shown in logfiles.

What can cause such behavior?