There is indeed an issue, but it does not raise an error message. The problem was in the wp-content/plugins/imi-classifieds/js/add-dynamic-fields.js file.
One of the problems is that this file is loaded across all the back-end, which is careless to do, as it's only used on the certain plugin settings page. It's a common mistake plugin authors make.
The second is that it re-sets the sortable items, because the search uses the same sortable class on the items. The plugin should however use a more specific selector to make sure that other sortables are not overridden. This might cause issues with other plugins as well in the future.
Anyways, I've opened the file in the plugin editor and made a change to that plugin file that might solve this problem. You might need to check it though, I'm not sure if that's effective. (make sure to clear your browser cache first)Best,
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