during my backup, rsync exits without finishing its job. i've upgraded rsync to version 302, and debugged rsync with the rsync-debug script. 
Here is the output from the last run: 

fstat(3, {st_mode=S_IFREG|0644, st_size=38452, ...}) = 0
read(3, "\37\213\10\0\327\316\17H\0\3\354]\331r\333H\262}\266\277"..., 38452) = 38452
select(2, NULL, [1], [1], {60, 0})      = 1 (out [1], left {60, 0})
write(1, "\374\17\0\7", 4)              = -1 EPIPE (Broken pipe)
--- SIGPIPE (Broken pipe) @ 0 (0) ---
write(2, "rsync: writefd_unbuffered failed"..., 76rsync: writefd_unbuffered failed to write 4 bytes [sender]: Broken pipe (32)) = 76
write(2, "\n", 1
)                       = 1
rt_sigaction(SIGUSR1, {SIG_IGN}, NULL, 8) = 0
rt_sigaction(SIGUSR2, {SIG_IGN}, NULL, 8) = 0
write(2, "rsync error: error in rsync prot"..., 87rsync error: error in rsync protocol data stream (code 12) at io.c(1543) [sender=3.0.2]) = 87
write(2, "\n", 1
)                       = 1
select(0, NULL, NULL, NULL, {0, 100000}) = 0 (Timeout)
exit_group(12)                          = ?
Process 7366 detached