Java Web Vararg Demo Displays an Empty Page Instead of the WebFigure

I want to deploy a Matlab script to the web in form of a Java app. I follow the instruction: Display a MATLAB Plot on the Web using a Java Servlet

The servelet is deployed in tomcat9 on Ubuntu Linux 18.04 and runs, but when I enter numbers and click on "Display Plot", a blank page is displayed.

It seems like that the web-part is working, but the Matlab part is either not executed at all or the output is not converted into a web-page.

The log file /var/log/tomcat9/catalina.2020-04-02.log does not contain any error message:

02-Apr-2020 21:13:28.548 INFO [Catalina-utility-2] org.apache.catalina.startup.HostConfig.deployWAR Deploying web application archive [/var/lib/tomcat9/webapps/VarArgServlet.war]

02-Apr-2020 21:13:29.289 INFO [Catalina-utility-2] org.apache.jasper.servlet.TldScanner.scanJars At least one JAR was scanned for TLDs yet contained no TLDs. Enable debug logging for this logger for a complete list of JARs that were scanned but no TLDs were found in them. Skipping unneeded JARs during scanning can improve startup time and JSP compilation time.

02-Apr-2020 21:13:29.301 INFO [Catalina-utility-2] org.apache.catalina.startup.HostConfig.deployWAR Deployment of web application archive [/var/lib/tomcat9/webapps/VarArgServlet.war] has finished in [754] ms

Any help to fix this is appreciated.

[Kojiro Saito]

Did you get any error message from Web browser's console?

For example, Firefox and Chromium have Console Window, so you can find any clue.

https://developer.mozilla.org/en/docs/Tools/Web_Console/Opening_the_Web_Console