I recently came into the possession of a DS920+. This was a replacement for my DS1010+. While switching Synology devices i also switched cloud providers. I found out the hard way that Synology Cloudsync was not performing as expected. Failed transfers did not seemed to be logged, and 1 out of every 2 files i tried to upload failed.
Install docker on your Synology, and use this docker image
This solution seems to work better for me. I also send a ticket to the Synology support. They were very cooperative, and they also told me that there could be problems with the Cloud Provider, which i later confirmed seeing the logs in the multiple docker containers i am running to sync the good stuff to the cloud.