I'm sorry that I asked you to try the URL, because it can't be reached from outside our firewall.

"hello.php" has the permission 644 and is owned by ftpuser:www-data
"index.php" has the permission 644 and is owned by ftpuser:www-data

apache is running as ftpuser:www-data

There is no difference between the permissions of "hello.php" and "index.php", so could the problems be somwhere else?

Try to put chmod 777 to check the permission.
I don't know if you have a rule somewhere else to forbid the access to the index.php.
try rename hello.php into index.php to try identify if there is another rule that forbid the access to index.php.

Contact the administrator of your server to assist you in the permission issue.

There is for sure a difference but you don't see it and it is very difficult for me to guide you without seeing anything.

I renamed "hello.php" to "index.php", and it works fine.

Could the problem be the content of "index.php"? Could there be any configuration files missing?

The administrator of this server is not available for the time beeing, but I am familiar with permissions in linux.

This just demonstrate the permission problem.

Edit the original index.php text and copy it into the "hello.php" renamed into index.php file.
You will have the permission of the hello.php instead of the one of the index.php file.

You have also to check the permission of ALL the files and folders present in the installation directory.

If you are famillar with permission and ownership and group of files and folders you should be able to find the reason yourself.
This is a problem of administration to give permission to execute the installation.

I will show you the file listing.
Can you please tell me what's wrong here?