Contact Widget doesn't work

Hi,

I just downloaded, installed and activated the contact widget plugin to a multisite using the Nelo theme. Next I wanted to drag it into the sidebar to set it up. It could not be dragged, didn't move. What's interesting is that at that time none of the other widgets didn't move either, I wasn't able to drag any of them. Once I deactivated the contact widget, all the other plugins were movable again. Very strange. Can you please help?
Thank you.