I have a problem in the Exam mode, when creating an essay type of question, where they have to upload a pdf file. For some students, the file uploading box is not appearing, and only a loading circle shows up, but never the box. They tried with all the internet browsers, and even with different computers and still doesn't work. Another issue that they have, is the math display is all messed up. Here is picture of how it looks, this was taken from a cell phone, but it's the same in several computers and browsers. 
Dario, have you installed moodle using script installer?