I really appreciate your reactivity, as usual.
I sought the warning for the first time when I was developing a theme for a blog that is not suppose to be multilingual and where I'm not using the plugin (The Lupita Project).
I suspect the problem could be related with another plugin that is central on this particular project, the Event Organiser plugin. Actually, the warning is only appearing when I'm editing an event (something like a post) in the dashboard, and exclusively in the view for a quick edit. The message of warning is repeated many times. I take a picture of the screen that you can temporarily see here.