Plugin throwing Soft 404 errors

Hi,
Our Google Webmaster tools Search Console says that we are getting loads of “Soft 404” errors. It lists URLs that I can’t decipher.

Based on the URL details and the HTML of our pages, I believe that they are coming from the Facebook Feed WD plugin.

Hoping that someone can point me in the right direction. Our website is here:
http://www.councilofindustry.org/

The URLs shown in the Google Search Console look like this (for example):

http://www.councilofindustry.org/wp-admin/admin-ajax.php?action=PopupBox&current_view=0&fb_id=1&theme_id=1&open_with_fullscreen=0&open_with_autoplay=0&image_width=800&image_height=600&image_effect=fade&enable_image_filmstrip=0&image_filmstrip_height=70&enable_comments=1&enable_likes=1&enable_shares=1&enable_author=1&enable_name=1&enable_place_name=1&enable_message_desc=1&enable_image_ctrl_btn=1&enable_image_fullscreen=1&enable_object_info=0&slideshow_interval=5&enable_image_facebook=0&enable_image_twitter=0&enable_image_google=0&current_url=name%3D3d-printing-goes-hollywood&image_id=137&image_id=

Following that URL simply gives a page with the error message “The image has been deleted.” No 404 error message or anything like that.

I’m guessing that this PARTICULAR error was generated on the blog post here – but I may be wrong about that; it might only be REFERRING to this blog post:
http://www.councilofindustry.org/3d-printing-goes-hollywood/

Since the URLs refer to a “Popupbox” I’m guessing(??) that this may possibly be related to the plugin’s “Lightbox” feature? As far as I can tell, we do not use the lightbox feature at all. I’ve tried to turn it off… On the options page for our Facebook Feed WD feed (a “page” feed), I’ve found that the lightbox “tab” is hidden. I’ve tried un-hiding it, and then setting all of the options to “No”, but I don’t think that has helped.

Why are we seeing these errors?
Why is Google indexing these URLs; or maybe they are not indexing them, but everytime someone views a page, ajax tries to load these URLs???
How can I correct this?
If I can’t correct them, what can I do to have those directed to a proper 404 error page?

ANY suggestions that might lead me in the right direction would be helpful!

Thanks!

The page I need help with: [log in to see the link]