Copy to - no default document library found in this site

Hi,

I have tried to copy a file to a new document library using the 'copy to' function. 

Usually this brings up a bar on the right of the page. Thus no longer happens and a dialogue box now opens with my sites down the left hand side. but when i click on any of them i get the following error:

Why has this system changed? and how do i fix the issue?

Best regards

Sam