Apache won't proxy to Tomcat 10

I'm installing a JSP webapp on Ubuntu 24.04 using Tomcat 10 and Java 11. The app is designed to have Apache proxy JSP requests to Tomcat via AJP. It has worked with previous versions of Ubuntu, Tomcat and Java going back to at least 2014.

When I attempt to access the index page of the app on the new system, the server returns a "503 Service Unavailable" error. As far as I can tell, I'm setting it up exactly like previous instances that have worked.

• Apache's mod_proxy and mod_ajp are both enabled:

$ ls /etc/apache2/mods-enabled | grep "prox"
lrwxrwxrwx 1 root root 32 Mar 17 13:02 proxy_ajp.load -> ../mods-available/proxy_ajp.load
lrwxrwxrwx 1 root root 28 Mar 17 13:02 proxy.conf -> ../mods-available/proxy.conf
lrwxrwxrwx 1 root root 28 Mar 17 13:02 proxy.load -> ../mods-available/proxy.load

• Tomcat's /etc/tomcat10/server.xml has its AJP Connector enabled:

    <Connector protocol="AJP/1.3"
               address="::1"
               port="8009"
               proxyPort="443"
               redirectPort="8443"
               maxParameterCount="1000"
               />

• Apache is configured to proxy to Tomcat:

<VirtualHost 139.93.241.16:80>
    # Server Name                                                             
    ServerName webapp.com

    # Tomcat proxy Information                                                
    ProxyRequests Off
    ProxyPassReverse  /        ajp://localhost:8009/

    ProxyPassMatch ^/(.*)\.jsp$    ajp://localhost:8009/$1.jsp
    ProxyPassMatch ^/$    ajp://localhost:8009/

</VirtualHost>

• Apache and Tomcat are both running.

If I comment out the Proxy directives in the Apache configuration, the URL http://webapp.com shows the Ubuntu Apache default page, as expected. If I directly access Tomcat using http://webapp.com:8080, I get the default ROOT/index.html page of the Tomcat webapp. This shows that both Apache and Tomcat are working individually. It's only when I enable proxying in the Apache config that I get the 503 error instead of the default webapp home page for http://webapp.com.

When I attempt to load http://webapp.com, the Apache error log at /var/log/apache2/error.log gives this error:

[Fri May 09 14:04:36.608181 2025] [proxy:error] [pid 273783] (111)Connection
refused: AH00957: AJP: attempt to connect to 127.0.0.1:8009 (localhost:8009)
failed
[Fri May 09 14:04:36.608221 2025] [proxy_ajp:error] [pid 273783] [client
110.71.264.19:37120] AH00896: failed to make connection to backend: localhost

I've searched for these errors, but they are so generic that I haven't been able to find anything relevant to this case.

The Tomcat error log at /var/log/tomcat10/catalina.out shows no records related to these attempts to access the resource, which is consistent with it not receiving the proxy request from Apache (I've configured systemd on Ubuntu to send Tomcat logs to catalina.out instead of journalctl).

What am I missing? What's the next thing I should check to troubleshoot this issue?