Dear Sir/Madam, I don't know exactly when the problem started to occur, but I've just found out that the MediaBox popup component doesn't work anymore.
If I click on the image nothing happens, and the problem occurs with the new and the previously created images+popup.
I've installed all the updates of JCE and I also have the latest version of MediaBox.
As far as I remember I haven't changed any options in the profile or global configuration.
I'll provide a couple of example:
Bottom of the page (footer), the QR Code should popup the 100% size when clicking:
Go to the bottom of the page, the popup image is the QR Code of the site.https://www.iisbassanoromano.it/
A previously created article in which there is an image that should popup the original image in full size:
Tornei sportivi scuola
Thanks for your answer in advance.
Dario Brama

The only thing I can think of is a new plugin which interferes with all "popups" scripts.
Can JQuery in a plugin (enabled or disabled) be the cause?

I know it may sound stupid but I reinstalled my template (Joomlart JA Coach + T4 System plugin) and everything's working now. Thanks for your help and explanation.