Problem on Modification of a prospect sheet

Problem on Modification of a prospect sheet

Hello,

I have a problem since this afternoon. 

When I view a  prospect sheet in the "description" tab, no change is possible, and when I try to change then I click save, I get the following on a pop up error message: "error: null ".

I tried to restart, but to no avail, this still does not work.

Thanks in advance for your help 

Best regards