It’s happened a few times now and I have had to death at all of the plugins, then one by one activate them again, the menu then works for a week or so then stops again.
You have a JS error that appears to be related to Autoptimize and you’e using a cache plugin. If you disable all active plugins and the drop-down menu works for a while. I would say this is related to your caching plugin.
I would recommend you check the cache plugin settings. Unfortunately, I only use WP Rocket and have no experience with ZenCache.