We have a test site running for our Company @ one.com where the converting of order data from the order.php file is working correctly. When I use the same file on the production site, which is hosted @ 1und1.de then the formatting of the files in CZ, SK and other languages fail

Does anyone have any idea, what could cause this?
File starts like this:
$file_encoding = 'UTF-8';
and Ends like this:
$ce = mb_convert_encoding($finalstr,'utf8');
fputs ($handle, $ce);
And as said, it works perfectly on the test site, hosted by one.com