With tomcat11, vnu webapp is reachable at:

  http://localhost:8080/vnu

On some architectures (eg. i386, loong64), vnu needs the JVM to be invoked
with the following JVM option "-Xss%SIZE%k".
Where %SIZE% is
  - 2048 on i386
  - 3072 on loong64

To achieve this, you could set the CATALINA_OPTS environment variable this way.
  SIZE=2048 && echo -e '[Service]\nEnvironment="CATALINA_OPTS=-Xss'$SIZE'k"' | \
    systemctl edit --drop-in vnu-tomcat11.conf --stdin tomcat11.service

Then restart the service:

  systemctl restart tomcat11

To allow testing a URL on localhost (or 127.0.0.1 or alike), add this to the
CATALINA_OPTS (like above): -Dnu.validator.servlet.allow-forbidden-hosts=true