My suspicion is that the ä in 'März' is in UTF-8 and not in ISO-8859-1/ANSI in the $SITE['monthNames'] = array(
'Januar','Februar','März','April','Mai','Juni',
'Juli','August','September','Oktober','November','Dezember'
);
in Settings.php.
The März has to be translated to March in order for the strtotime() to be able to handle the month name.. since it can't translate it by lookup, the strtotime() returns a 1970 date for the local date. Try editing your Settings.php and convert it to ISO-8859-1/ANSI and retype the ä in März and upload your Settings.php again.