To make the Simple Portfolio plugin (https://wordpress.org/plugins/simple-portfolio/) working via https (or with a ssl certificate) you must change some files in the source code.
Edit the following files:
- /extends/admin-head.php
- /extends/admin-menu.php
- /extends/init.php
- /extends/save-post.php
and replace
WP_PLUGIN_URL
with
plugins_url()
Discussion here: https://wordpress.org/ideas/topic/wp_plugin_url-doesnt-take-ssl-into-account