Admin Panel Issue (Slides) – Custom Theme

We had a custom theme built in 2012 and the developers have since moved on. The site works but under the hood there is a problem: I cannot DELETE SLIDES in the custom Slide section our WP Admin panel.

I’ve looked to see if anyone had similar problems and its a common occurrence, usually resolved by the plugin developer, or the topic is closed or unresolved.

The slider works fine, but in the admin panel I cannot delete any individual slide. When you click “TRASH”, it says “1 item moved to the trash”(
Image 1)

However, there is no trash folder and the trashed slide goes nowhere. When you click “UNDO TRASH” you get an error message (Image 2)

WP & plugins are updated, currently using 4.3.1, and caches cleared.

To ensure it was a code error and not a browser error, I tried deleting the slides in Firefox, Chrome, and Safari, and failed in all 3 attempts, so the issue is not browser specific.

Then I enabled script debugging: define(‘SCRIPT_DEBUG’, true);
and the issue persists so now I’m unsure of what to do, where to go, or what further steps to take.

Is this a needle in a haystack? Or have I overlooked something? Did the previous developer put a fail-safe on the code that I can’t see? I’m pretty good with direction so if anyone has any suggestions or knows what to do, by all means please point it out.

Thank you.