if i use EthernetClient.write() it works fine, code gets a bit ugly because the String class don't have a function to gain direct access to the internal buffer.
The String class has a toCharArray() method to get the whole array and a charAt() method to get one character. Use a for loop to get one character at a time, or make a copy of the whole buffer.
Better yet, quit using the String class. People wrote extensive text processing applications for decades before the String class wrapped the C code that really does all the work.