Hello,

Indeed as Mohsen said you need to first check your Apache/Nginx error logs. The default location of the Apache error logs on CentOS is:

/var/log/httpd/error_log

What you could do is grep for your domain name in the error log:

grep your.subdomain.com /var/log/httpd/error_log

That way you would get all of the entries from the error log for this particular subdomain name.

Hope that this helps!
Regards,
Bobby