The issue was caused by some missing Style Sheets on the server side, and occurred only when the displayed language was different than the main language.
If you go on the Server installation folder and navigate to \CRMWeb\_static\css, you will see a folder for each of the languages you have installed. In my case there were two folders, 1033 (english) and 1040 (italian). The processControl.css file under folder 1040 (main language) was bigger then the one under folder 1033 (alternative language): i inspected it and found that it missed several lines of code!
How to solve it? Just copyy processControl.css from your main language folder and replace the one on the other language folders. After reloading the browser, all the Business Process Flows will render correctly.