PhpStorm fails to validate Debugger on remote server

Question

I'm deploying my website to a remote hosting server. The provider says it has the Zend debugger enabled. In the phpinfo I find all these lines that refers to Zend:

Zend Signal Handling    enabled
Zend Memory Manager     enabled
Zend Multibyte Support  provided by mbstring 
report_zend_debug   On  On
zend.assertions -1  -1
zend.detect_unicode On  On
zend.enable_gc  On  On
zend.exception_ignore_args  On  On
zend.multibyte  Off Off
zend.script_encoding    no value    no value
zend.signal_check   Off Off

In PhpStorm I create a PHP Remote Debug configuration (and also a PHP Web Page) but when I try to validate the debugger it successfully deploy the script _intellij_phpdebug_validator.php but it fails with this error:

Validation script was created but cannot be executed. Possible reasons: 
Configured 'URL to validation script' is incorrect
Validated Web Server is not running.
Target directory for validation script is not public.

1. The URL is the root of my hosting space, say https ://mydomain.com. Opening this with a browser renders the homepage
2. The Web Server is running because as said the website is up
3. The target directory is at the same level of the index.php so it's public (i.e. PhpStorm deploy files there)

Furthermore, during the validation Filezilla shows me the _intellij_phpdebug_validator.php on the remote server, so it actually deployed it.

What I need to configure further to enable a remote debug?

UPDATE

The hosting provider let me view to errors log only and there I don't find anything related to the validation script.

Manually accessing to the script from a browser leads to this content:

<?xml version="1.0"?><validation><path_to_ini path="/php7.4/etc/php.ini"/><additional_php_ini files=""/><server server_name="<mydomain>" remote_addr="<my public IP>"/></validation>