Solved by myself after looking a bit around.
The PHP’s slow log was telling the script was stopping at line 1032 of wp-content/plugins/geodirectory/geodirectory_template_actions.php, in the getimagesize() function.
I have added a line before such command:
error_log("Image is: ".$image, 3, "/tmp/wordpress.log");
and ran the listing update. Then I took a look inside /tmp/wordpress.log:
Image is: https://myfqdn/wp-content/uploads/2015/12/temp_1/aerialview1.jpg
and well… doing a:
curl https://myfqdn/wp-content/uploads/2015/12/temp_1/aerialview1.jpg
from the very own server got stuck.
TL;DR: the myfqdn IP address was resolving to the external, non-reachable IP address because of DNS resolution 😛 Put myfqn as 127.0.0.1 in /etc/hosts and voilà!