popup loading on each subsite of my wp multisite installation

Since few weeks I am seen a popup windows on each of my subsite.
I can't understand why. I am using only google adsense and don't think this is it.
I don't like popup windows. Please can you help me to understand where this is coming from? I want to delete it and avoid this popup in the future. Is this a virus? Or a plugin that I am not aware of?
Please feel free to have a look at this subsite:
or any subsite of the mainsite

support access is granted.

Thank you very much.