Recently I was trying to download the access logs of a Glassfish installation via ftp. I was unable to do so and was getting this weird error. I guessed it might be a permission issue and logged into the server with remote desktop. Everything turned out to be fine and all valid permissions were in place. A quick search on Google turned up this FAQ on Filezilla Wiki:
FAQ: I am having trouble transferring log files. I get an error “550 can’t access file.” on both the server interface and the FTP client.
Answer: Often log files are locked for writing by the process that they are logging. In order to transfer this type of file, open the Server Options, go to the Miscellaneous node, and select “Allow downloading of files which are open for writing by another process”
It probably isn’t a FAQ, but the FAQ is the only server documentation you have so far.
So there you have it… Check your server setting first before tearing your hair out.