I've had this problem for months, and kept coming back to my world to try gain and again. I realised that it was letting me download "World", but not the worlds I uploaded initially onto the server. So I just got the WorldEdit plugin, made a giant selection of the city we made, saved into schematics and pasted it into a newly generated world (using //paste -o ) with the SAME seed . It crashed the server a few times and I had to manually reset it but eventually it stuck and I managed to /dl the world. N.B. It might be worth getting the multiverse plugin to make a backup of your world first before you do things like worldedit or reset world/force hibernate.
