The actual full error on my system is:

2015/06/03 12:47:17 [2012] rsync: read error: Connection reset by peer (104)
2015/06/03 12:47:17 [2012] rsync error: error in socket IO (code 10) at io.c(785) [sender=3.1.1]
2015/06/03 12:57:18 [316] sent 0 bytes  received 0 bytes  total size 0

Inside the BackupPC XFER Error Log:

Sending args: --server --sender --numeric-ids --perms --owner --group -D --links --hard-links --times --block-size=2048 --recursive . .
Xfer PIDs are now 25110
[ skipped 5027 lines ]
Read EOF: Connection reset by peer
Tried again: got 0 bytes
Can't write 4 bytes to socket
finish: removing in-process file path/sanitized/for/your/viewing/pleasure/UVHD-S-1-5-21-773923596-1703164468-81712224-1105.vhdx
[ skipped 7 lines ]
Child is aborting
Done: 231 files, 38884879 bytes
Got fatal error during xfer (aborted by signal=PIPE)
Backup aborted by user signal

You'll notice this is a VHDX file - 40GB in size this one is. The problem is indeed a timeout, but it's not at the BackupPC end, it's at the rsyncd end. I added the following line to the proper module of my rsyncd.conf on the host being backed up:

 timeout = 10800

Problem remains unsolved as of yet. It seems the problem is not necessarily an rsync copy problem but they way that it's choosing to compare the files, or maybe the business with copying deltas. My current workaround is to ignore those files since they're not in active use anyway. It should be noted their first full backup went without a hitch.