Reading the stack trace and my guess:
java.net.SocketException: Connection reset
This means that your script (the client connection) was trying to read something from the (the server connection – look like a SOAP invoke based on com.niku.union.xml.SimpleSOAPMessage.processReques) but when reading the response (java.net.SocketInputStream.read) it either tried to read past the EOF or it was closed.
My gut call (without see the script) is that your SOAP request is taking some time and your client gets tired of waiting for an answer, closes the connection and moves on.
You could look at the system log to see what the server was doing at around the same time of the connection reset to see if this gives you any clues on what might happening.
V/r,
Gene