'please define pop up taxonomy' message


I am running a WP network providing (among others) wpmudev PopUp Pro plugins to subsite admins who need it.

One of them who activated the PopUp Pro plugin has now this warning message on his dashboard:

Please define pop up taxonomy by adding define("POP_UP_TAXONOMY", "custom_taxonomy_name"); in your wpconfig.php file.

Does it come from the PopUp Pro plugin ?

And if so, I don't understand why he gets this message whereas other users of the plugin don't ?

Thanks for your advice.