Tried your site but could see no problem specifically! See pic empty-cats01.png which shows files present, this is same location as you sent.
I am sure you have had this site running some time quite successfully so something must have changed!!
I could download the first item AOK (Mediterranean Strategy on Education for Sustainable Development) but the second one produced an interesting message from your server - see pic empty-cats02.png So perhaps someone has been changing directory permissions. Also on another instance I got a server fault about some problem your hosting company was working on fixing.