The Resilio Sync package that’s provided on a Synology NAS’ Packager Center is out of date (2.6.2) and has a bug that causes big files to fail to sync (and that causes a loop of endless retries).
Note that you need to stop the Resilio Sync service running (manually stop it via Package Center) before uploading the package and installing it.
Once done, don’t forget to manually start the service again.
Resilio Sync GUI will be at
The howto guide to manually update the package can be found here.