Make sure your files have been saved as UTF8 without BOM. For that, you need an editor that is capable of saving files as UTF8 without BOM, like Notepad++. The standard Windows editor is not capable of this.
Also when editing PHP-Files, make sure that there is nothing before "<?php" and after "?>".
If that doesn't help, try re-uploading the language files of phpBB Statistics and the ones you modified.
And as always, refresh the board cache.