The SG automatically resets Schannel when it gets an unexpected error code back from the DC.
This error code refers to LW_ERROR_RPC_NETLOGON_FAILED and will cause an Schannel reset. That error is unexpected, and could mean that the port 445 TCP connection between the SG and the DC (Domain Controller) was interrupted. That could mean there was a problem with the DC, or it could be due to some device between the ProxySG and the DC such as a firewall occasionally dropping connections.
A packet capture on "tcp port 445 or tcp port 139" should help in investigating the problem further.