I created a duplicate of the site to test on the same environment. I have deactivated the theme and activated the 2017 theme and also the plugins from the theme. The issue is gone, so it must be theme related.
I have also updated the leader theme to version 2.4 and the issue is still there.
Site URL: hidden Theme version: 2.4 WooCommerce version: 3.1.2
Content is visible for topic creator and support staff only.