Hmmm, I cannot seem to replicate the error you getting when trying to delete slides, and I’ve just tried on 2 separate test sites.
Try to see if you are getting some sort of JavaScript error by opening up your browser console (press CTRL + SHIFT + J together) and then performing a ‘Delete Slide’ and then see if any error message displays within the console. If so then this will give me something to work on.
Also try editing your ‘wp-config.php’ file (in the root folder of your site) and change the ‘WP_DEBUG’ line to ‘true’ and then try deleting a slide. This will shor any PHP errors occurring. Change it back to ‘false’ once done testing.