What version of rsync are you running on the client?
Another user reported a problem that the default cygwin rsync setup on 3.x doesn't allow remote charset settings. BackupPC 4.x uses the --iconv rsync option, which will fail without a clear error message.
You should add "charset = utf-8" to the client's rsyncd.conf to see if that's the issue.
(Since it fails very early in the transfer, I'd recommend you add "-vvv" to $Conf{RsyncArgs}.)