I have made some progress with a workaround.

I copied (cp -r -p) the folder that was successful to the virtual site that it was intended for.
Deleted the config file, created a new database and restarted the install, after downgrading the "optional" file permissions (owner Apache 0744 & 0644).
I cannot be sure if this was the reason, because when I attempted a clean install from zip, it was not successful.

Because I am not familiar with "find -exec chmod", I needed to deal with it with WinSCP.

Please advice.

Edit:
I discovered a problem with the fake users ... most are not selectable.
I deleted all of them and generated new fake users. who do have a profile.

I tested a dummy domain and I get the same result.

John wrote:

I didn't change the webserver from Sandbox to Standard, because that means I have to remove it and recreate it ... there is no option to change it.
I installed it in a Sandbox successfully once and I don't want to screw up my directory structure, because I have more virtual domains and prefer to keep them separate.

As you can see in the pictures, I already tried it with CGI enabled, but with the same result.

If PHP 7.4 is not possible from COS then I will have to keep trying it with pH7Builder 16.2.2 instead of 17.x.
Any of the yum commands result in "Nothing to do", but even if it did work, I would not be able to setup my webserver to use it.

# php -v

PHP 5.4.16 (cli) (built: Apr  1 2020 04:07:17)

Copyright (c) 1997-2013 The PHP Group

Zend Engine v2.4.0, Copyright (c) 1998-2013 Zend Technologies

Please advice.

I'm not using sandbox, so i'm quit sure if PHP-engine is working in it
But you can select in the webconfig/webserver the php version for each specific site

See screenshot.
I've for this site php 7.3 running

The php 5.4.16 is for webconfig and server

You can try to add a new website and select "standard" and then install the software to test
This will leave the others site in tact

I didn't change the webserver from Sandbox to Standard, because that means I have to remove it and recreate it ... there is no option to change it.
I installed it in a Sandbox successfully once and I don't want to screw up my directory structure, because I have more virtual domains and prefer to keep them separate.

As you can see in the pictures, I already tried it with CGI enabled, but with the same result.

If PHP 7.4 is not possible from COS then I will have to keep trying it with pH7Builder 16.2.2 instead of 17.x.
Any of the yum commands result in "Nothing to do", but even if it did work, I would not be able to setup my webserver to use it.

# php -v

PHP 5.4.16 (cli) (built: Apr  1 2020 04:07:17)

Copyright (c) 1997-2013 The PHP Group

Zend Engine v2.4.0, Copyright (c) 1998-2013 Zend Technologies

Please advice.

Dit you change the webserver from Sandbox to Standard ?
You can enable CGI with the Webserver config.

PHP 7.4 is not possible from Clear.

Maybe this will help
https://computingforgeeks.com/how-to-install-php-7-4-on-centos-7/

Not sure howto enable it from the webserver

Of course I checked the requirements and for version 16.2.2 it seems to be ok, because I installed it successfully once as you can see in the pictures and no error messages were reported during any of the installs.

But to be complete I will reply to all you wrote:
Application Server PHP 8.0 or newer (optimized for PHP 8+ and beyond! ?).
For version 17 & higher, PHP 7.4 is required, but I would not know how to upgrade PHP on a COS box.

Database MySQL/MariaDB 5.5.3 or newer.
Server version: 5.5.68-MariaDB - MariaDB Server

Operating System Linux/Unix (Red Hat, CentOS, Debian, FreeBSD, Mandrake, Mac OS, etc.), Windows.
ClearOS 7.9.1

Web Server Apache with mod_php or with PHP in CGI, FastCGI mode.
PHP 7.3.29 (Is it even possible to upgrade to 7.4 on a COS box ... ?!?) & CGI enabled

URL rewriting extension module Apache.
As you can see I enabled Follow Symlinks / Allow Rewrite

Specific Requirement Server has to be connected to Internet.
Of course

Minimum Web Space 2.0 GB
Plenty

pH7Builder's Video Module Requirement (only if enabled) FFmpeg
FFmpeg is installed by default on a COS box.

Please advice.

Did you check the requirements?


Requirements

• Application Server PHP 8.0 or newer (optimized for PHP 8+ and beyond! ?).
  
• Database MySQL/MariaDB 5.5.3 or newer.
  
• Operating System Linux/Unix (Red Hat, CentOS, Debian, FreeBSD, Mandrake, Mac OS, etc.), Windows.
  
• Web Server Apache with mod_php or with PHP in CGI, FastCGI mode (nginx, LiteSpeed and IIS should also work. You might have to change some pieces of code and change the URL rewriting to make it work).
  
• URL rewriting extension module Apache, nginx, LiteSpeed, IIS (for Web.config, you have a good tutorial here).
  
• Specific Requirement Server has to be connected to Internet.
  
• Minimum Web Space 2.0 GB
  
• pH7Builder's Video Module Requirement (only if enabled) FFmpeg
  

Try to make the website as "Standard" instead of "Sandbox".



Problem can be in the PHP8.0 because the COS is using PHP7.4