(54: Connection reset by peer)
What does this message mean :-[ :-[
[b]Apr 26 19:31:46 pfsensefirewall.localdomain nginx: 2016/04/26 19:31:46 [error] 24577#0: send() failed (54: Connection reset by peer)
Something on the other end sent a reset (RST).
e.g. this explanation: http://stackoverflow.com/questions/1434451/what-does-connection-reset-by-peer-mean
So the "something" did not go away completely (unplugged, powered off…) - it actually sent an RST (and went away).
What Phil said. Specifically, something that was communicating with the web GUI's web server. Likely normal and safe to disregard.