…as indeed it did.

I am not saying with 100% certainty that another plugin is the cause but I have to say that whenever we have encountered similar – though rare –  behaviour before t has to date always come down to one. And some of those plugins you are using have not been updated for quite some time.

Of course I don’t know where to start as I have never encounter many of those plugins before. I would, perhaps, try deactivating both of the light-box WP plugins just to see if that has any effect. You can reactivate them immediately of course,