Resilio Sync on Synology stuck?
The Resilio Sync package that's provided on a Synology NAS' Packager Center is out of date (2.6.2) and has a bug that causes big files to fail to sync (and that causes a loop of endless retries).
The solution is to manually update the package from 2.6.2 to 2.6.4. Find your NAS architecture and then download (bottom of the page) the right binary and manually install it.