After enabling the messenger v3 service you just get the default Apache page when testing the messenger v3 service. You will also see this in the default Apache logs:
[Thu Aug 24 04:31:24.428103 2023] [authz_core:error] [pid 91755:tid 139645249562368] [client 192.168.1.133:60254] AH01630: client denied by server configuration: /home/csf/public_html/
This can usually be fixed by adding the line Require all granted to the beginning of the .htaccess file at /home/csf/public_html/.htaccess
Require all granted
DirectoryIndex index.php index.cgi index.html index.htm
#Options +FollowSymLinks +ExecCGI
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^ /index.php [L,QSA]
CloudLinux
CloudLinux users should also disable CageFS for the csf user:
cagefsctl --disable csf