Impossible to add a widget to the sidebar

Hi,

Our server went recently to crash, and I am now reparing our site www.voilesdantan.org.
My problem is that I cannot add any new widgets (meta or minimeta, wp PHP, ...) to the sidebar : when I move them there they are not activated, and they disappear when I refresh the page.
I connot find out what is blocking the insertion of widgets.
Note that I have access to the existing widgets (search, events-calendar, nextgen, google weather...)
Hope that someone has an idea ? Thanks for help.
Manu