Thanks again, Carsten, you're comment was most helpful.
I checked the Apache logs and found the error.
The problem was the ".htaccess" located in LS_DIR/classes. This file contains only one line, which is "options -indexes".
My Apache configuration is set up to disallow overriding the options directive via a htaccess file. That's why the ".htaccess" file caused a 500 error.
Maybe the ".htaccess" file in question were better removed from the distribution? I mean, virtually no other directories in the LimeSurvey distribution have a htaccess file.
Bye, Malte