Hi, I had the same error message on my installation. I checked my upload size limit and it was 128M but still the error ocurred. When I changed the permission to moodle folder to read/write, the error stopped.
So you can try it and see. It should work for you.
Thought I would post my findings so that when Google throws this up for someone that is using the Synology setup that it might save them a day..
I have detailed a post here for how to find it and fix it on our systems..
https://forum.synology.com/enu/viewtopic.php?f=190&t=87663&p=420405#p420405
