What do you mean by it doesn't work? Clicking the button does nothing? The upload fails and the page reloads? Please be more specific. If the button does nothing you may have some sort of fatal JS error present on the page. Press F12 to open your browsers developer tools and click the Console tab. Now refresh the page and attempt to install a plugin and see if any fatal errors (they'll be red) are in the console.

If the upload process itself isn't working then that would be an issue with your connection to the server. Opera nor Brave are tested browsers. We test Chrome, Firefox, and IE only. Try one of those as maybe browser security for whatever reason is blocking the upload for whatever reason. Using Chrome or Firefox perform the F12 console check if the upload also isn't working there to rule out JS errors blocking form submit. Aside from that you can try Install from Web assuming your browser security doesn't block the JSONP request preventing it from seeing your Joomlapolis login state.

Strange, that's an error thrown by qtip which we use for tooltips. It should only try to call that if the jQuery function imagesLoaded exists. I'm not seeing such an issue in my tests. Please PM backend super administrator login credentials and will take a look.