Basically it's the same issue as here, but the OP never posted his solution...
viewtopic.php?f=3&t=6450
Installed Community version of Document Server and I am attempting to create/edit a new file from ownCloud.
Installation and initial checks appear to complete ok but when opening the file in OnlyOffice, the error on screen is: "Download Failed"
and in the logs I see this: "type":"documentOpen","data":{"type":"open","status":"err","data":-81}
any clues as to what it might be ?