Error 500 exporting subscribers

Hello,
I’m etting error 500 when trying to export my subsrcibers.

I activated wp in debug mode and discoverd that the problems is caused in
wp-content/plugins/wysija-newsletters/classes/WJ_Export.php
line 182
$encoded_string = iconv( $this->_base_encode, $this->_output_encode, $row_string );
and line 204
$encoded_string = iconv( $this->_base_encode, $this->_output_encode, $row_string );

I manage to workaround this by commenting them out and using $row_string directly. However these causes problems with special character (UTF-8).

Can you provide help on this?
Thank you
Sérgio Rosa