Do you know if opera beta can displayed a site in mobile modality or in desktop modality?
If yes, how I can active/deactive this option?

I don't have a login for that site and don't plan to create one.

Ok, then you visit this link https://www.mediaworld.it/mw/mediaworld-club without login.
Is it well displayed?

I haven't had the page load incorrectly under either Windows or Mageia Linux ...

Did you visit this link https://www.mediaworld.it/mw/mediaworld-club or did you do only the login?
In the second case, after login, visit that page

Both computers use the same DPI?

I don't now. Anyhow, I have changed the resolution on the pc when the visualization is correct and the visualization continues to be correct.

I think the problem is resolution. Set both the computers to the same resolution and see if the problems persists

No. They are two different screens. The first have 19"; the second have 15.3". In any case, in windows, I have not this problems also with different resolution

The image of correct one shows that ad-blocker is disabled while the wrong one has it enabled. So I would starting by disabling ad-blocker on that site.

I have disable ad-block but the problem persist

We do have an Italian forum, if you'd prefer. Click on the globe icon below to change languages.

Because on the Italian forum there is nobody

Also, I can see from the icon you're using Beta, not Developer.

Excuse me. I'm wronged. Like you have say, my opera is opera beta and not opera developer

Don't recognize the OS, but it seems to be some version of Linux - knowing what might help here.

My operative systems are Debian Stretch (Stable) and Debian Buster (Testing)

I use opera developer on two different pcs. (same version) but the visualization of the site www.mediaworld.it is different. Why?

Correct vilualizzation: https://ibb.co/hCffNv

Wrong visualization: https://ibb.co/mots2v

Uso opera developer su due pc diversi (stessa versione) tuttavia la visualizzazione del sito www.mediaworld.it è diversa. Come mai?
Visualizzazione corretta: https://ibb.co/hCffNv

Visualizzazione errata: https://ibb.co/mots2v

Uso opera developer su due pc diversi (stessa versione) tuttavia la visualizzazione del sito www.mediaworld.it è diversa. Come mai?
Visualizzazione corretta
[url=https://ibb.co/hCffNv][img]https://preview.ibb.co/j0g9FF/2017_07_15_111623_1680x1050_scrot.png[/img][/url]
[url=https://it.imgbb.com/]images upload[/url]

Visualizzazione errata
[url=https://ibb.co/mots2v][img]https://preview.ibb.co/d5v5Nv/2017_07_15_112955_1366x768_scrot.png[/img][/url]
[url=https://it.imgbb.com/]image hosting[/url]

How do I see the opera's cache?
I've opera 40.0.2288.0