Hi Federico.
Restarting the kernel only will not solve the issue.
You need to follow the instructions I pasted above to get a clean version of the notebook, and then adjust it with your answers. This issue is happening because an object that is used in the grading was probably deleted by mistake.
Please follow the instructions above first, and you should get the full grade, considering you are saying that your output is the same as the “required output" 
I just completed the lab to see if there was another issue, and you can see that I got full marks from the screenshot below.
