You should check the error log of Apache. Error logs are usually located in /var/log/httpd/GROUP/ as DOMAIN-error_log and DOMAIN-ssl_error_log
GROUP is your Linux group name you can obtain via command “groups”.
[exam@webcloud01 ~]$ groups
Apache log for “exam” is under /var/log/httpd/ase/
Accounts and virtualhosts from the same original CentOS 6 webcloud server, i.e. the same department/unit, are in the same group, are able to view the log of the same group but not others.
Custom modifications and plugins. Even if your main software is upgraded to a compatible version, plugins and custom modifications may not be PHP7.3 compatible. To resolve, try to disable all installed plugins and modifications and then re-enable one by one until you find the plugin that causes the error. Help may also be in the error_log on your server
There may be special rewrite rules, redirection rules, or hardcoded PHP code/database record using the original domain name. Using temporarily domain name “nOriginalDomainName” may not work or may not make your test completely. Please consider to use “local host files method” to test.