unexpected tag -55

[Yakov Hrebtov]

Periodicaly rsync terminates with folowing errors:

unexpected tag -55 [receiver]
file has vanished: "<filename here>"

server log says:
file has vanished: "<filename here>" (in userdata)
writefd_unbuffered failed to write 4 bytes [sender]: Connection reset by peer (104)

I'm didn't wondered with vanished file, but what is "unexpected tag" and 
"writefd_unbuffered failed"?

What is the cause of this errors?
Is it network problems or something else?

Thank you.