Apache to authenticate web client with different methods and then proxypass request to Tomcat

I have an Apache webserver (frontend) and a Tomcat application server (backend).

I want to allow web clients to authenticate either with SAML or digital certificate and then proxify them to the backend server.

I'm almost sure the following configuration should work, but it doesn't.

I get error "Not Found. The requested URL was not found on this server."

and Apache logs says:

AH00128: File does not exist: /var/www/html/public/myapp/, referer: https://frontend-server/

Any help appreciated.

<VirtualHost frontend-server:80> 
        ServerName frontend-server
        Redirect / https://frontend-server
</VirtualHost>

<VirtualHost frontend-server:443>
        SSLEngine on
        SSLProxyEngine on
        SSLProtocol all -TLSv1.3 -SSLv2 -SSLv3

        SSLCertificateFile frontend-server.crt
        SSLCertificateKeyFile frontend-server.pem
        SSLCACertificateFile CA.crt 

ProxyPass /saml ajp://backend-server:8009/myapp
ProxyPass /cert ajp://backend-server:8009/myapp

        <Location /saml/>
                AuthType Mellon
                MellonEnable auth
                Require valid-user
                MellonVariable "cookie"
                MellonMergeEnvVars On ":"
                SetEnv MELLON_NAME_ID %{MELLON_NAME_ID}e
                RequestHeader set Mellon-NameID %{MELLON_NAME_ID}e
                ProxyPassReverse /myapp
        </Location>

        <Location /cert/>
                SSLVerifyClient require
                SSLVerifyDepth 2
                SSLOptions +ExportCertData
                ProxyPassReverse /myapp
        </Location>

</VirtualHost>