I just wanted to post this here for others to find via Google. If you're having problems with your Synology NAS (DSM) being able to complete updates, check your web filter settings. I was receiving a "Failed to download "CloudSync" error in package center, but you might also see a "connection failed" or "diskstation cannot connect to the internet" type error.
If you have "Download Freeware & Shareware" policies enabled, including warn, this will break the update functionality. The NAS is trying to connect to https://global.download.synology.com, which is (correctly) classified as freeware/shareware. I was pulling my hair out trying to track this down, and looking through firewall logs trying to figure out what was going on. Once I switched over to web-filter logs, the problem was immediately apparent. Add an exception for the aforementioned URL, and this will resolve the issue.
Hope this helps someone googling for a solution.
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