I am using BTSync for Synology.
I think there is a bug in the history log that shows the wrong message. If you start to sync a folder with another device for the first time, and the other device includes some of the same files that are in the source, instead of stating "File already found, skipping download" or something to that effect, the warning message in the log is, "Failed to download <filename> - CheckOriginalFile: No such file or directory".
Also, it would be very helpful in the History Log if the folder name was included along with the file name. Right now it just shows the file name that was synced, so you have no idea what folder it was from.
These things are pretty minor, but probably worth fixing.