getOutputStream() has already been called for this response

The issue here is that your JSP is talking directly to the response OutputStream. This technically isn't forbidden, but it's very much not a good idea.

Specifically, you call response.getOutputStream() and write data to that. Later, when the JSP engine tries to flush the response, it fails because your code has already "claimed" the response. An application can either call getOutputStream or getWriter on any given response, it's not allowed to do both. JSP engines use getWriter, and so you cannot call getOutputStream.

You should be writing this code as a Servlet, not a JSP. JSPs are only really suitable for textual output as contained in the JSP. You can see that there's no actual text output in your JSP, it only contains java.

Add the following inside the end of the try/catch to avoid the error that appears when the JSP engine flushes the response via getWriter()

out.clear(); // where out is a JspWriter
out = pageContext.pushBody();

As has been noted, this isn't best practice, but it avoids the errors in your logs.

Ok, you should be using a servlet not a JSP but if you really need to... add this directive at the top of your page:

<%@ page trimDirectiveWhitespaces="true" %>

Or in the jsp-config section your web.xml

<jsp-config>
  <jsp-property-group>
    <url-pattern>*.jsp</url-pattern>
    <trim-directive-whitespaces>true</trim-directive-whitespaces>
  </jsp-property-group>
</jsp-config>

Also flush/close the OutputStream and return when done.

dataOutput.flush();
dataOutput.close();
return;