There is a bug in MailPress 5.2 when you have several category newsletters released at the same time.
First newsletter is ok, following newsletters have an empty content.
Here is the fix :
in mailpress/mp-includes/class/MP_mail_.class.php
line #357 is :
function load_template( $_template_file, $require_once = true )
and must be changed in :
function load_template( $_template_file, $require_once = false )