Using relatedcontent.tpl.php in its default location (under the folder of the module) did not call my custom node-x.tpl files inside my themes.

What I did then was copying relatedcontent.tpl.php into my theme folder. This resolved the problem of not calling my custom node-x.tpl files.

However, this introduced a problem of not displaying the titles of the groups. I have checked and the function call 

relatedcontent_group_title($grouped, $group, $node_type)

returns an empty string now. This was not happening when the file relatedcontent.tpl.php in its original location was the active one.

I have checked the values of the 3 parameters sent and they are all OK:

$grouped == 'view'
$group == view-x
$node_type == simplenews

Any ideas hoe can I retrieve now the titles back?