I had the same issue creep in on a system that had been running fine for years. Ubuntu 14.04.3. My images directory (and all others) are owned by root:root, but have 777 as permissions, and fog could put ftp files to the directory and delete them w/out issue. However, I noticed that there was an extra line in /etc/vsftp.conf on the wiki pages than I had in my /etc/vsftp.conf.
seccomp_sandbox=NO
I added that line, and since I wasn’t sure how to restart the service, I just rebooted the machine. Success! I hope this helps someone else with the issue. I’m not sure how the change happened, but I bet it had to with ubuntu updates at some point. My first failed upload was in Oct, and I only now was able to circle back and address it.