Actually one thing that does come to mind is a possible ownership issue.
Make sure the uploads/ directory is owned by the correct user*:
# chown -R thecorrectusername uploads
*(On a regular web server I would suggest that the correct user is the same user account that the main website runs under. Of course, this doesn’t really apply to a Synology NAS so I’m not sure what to suggest in this instance).
I hope that this helps! Let me know.