Clearing cache in Drupal 8 on ISPConfig 3.1dev results in crashed site

Ubuntu 16.04
ISPConfig 3.1dev
Maria/MySql 5.6+
PHP 7.0
Drupal 8

We follow the instructions from https://www.howtoforge.com/tutorial/how-to-install-drupal_8-with-apache-and-ssl-on-ubuntu-15-10/ to set it up
We have multiple sites on this server. I've had an issue on two different sites on the same server. I have not maintained any of the other sites, so I haven't tested. After I have installed a theme (not the same one on both) if I clear the cache the site breaks. One the first one the pages are blank white.
The problem first started like this:
Theme Bootstrap. I’ve was unable to access the site. It worked the day before but today every page gave me a 404 error except the home page. The problem started on my PC and then later it started happening on the Mac.
I found a fix:
I went through this troubleshooting guide and apparently I needed to have “mod_rewrite” enabled. Of course. Duh. (and why did it work until today?)
https://www.drupal.org/node/228462
So I had to go here to figure out how to do that:
https://stackoverflow.com/questions/869092/how-to-enable-mod-rewrite-for-apache-2-2
Which had these instructions:
==============================
use the following .htaccess file.

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

The above .htaccess file (if placed in your DocumentRoot) will redirect all traffic to an index.php file in the DocumentRoot unless the file exists.

So, let's say you have the following directory structure and httpdocs is the DocumentRoot

httpdocs/
.htaccess
index.php
images/
hello.png
js/
jquery.js
css/
style.css
includes/
app/
app.php

Any file that exists in httpdocs will be served to the requester using the .htaccess shown above, however, everything else will be redirected to httpdocs/index.php. Your application files in includes/app will not be accessible.
================

So I tried that and it worked. But then when I cleared the cache after that (Performance > Clear cache) it gave me: "The website encountered an unexpected error. Please try again later."
So I removed the above code and now the pages are just blank.
If I go to install.php it says file not found.

That's the first one-- the second one where I had installed Zircon does this after I cleared the cache, for anything at all, login logout, anything:
"The website encountered an unexpected error. Please try again later."
I go to the install.php I get this:
Additional uncaught exception thrown while handling exception.
Original
Symfony\Component\DependencyInjection\Exception\ServiceNotFoundException: The service "password_migrate" has a dependency on a non-existent service "password_original". in Symfony\Component\DependencyInjection\Compiler\CheckExceptionOnInvalidReferenceBehaviorPass->processReferences() (line 58 of /var/www/clients/client1/web9/web/vendor/symfony/dependency-injection/Compiler/CheckExceptionOnInvalidReferenceBehaviorPass.php).

Symfony\Component\DependencyInjection\Compiler\CheckExceptionOnInvalidReferenceBehaviorPass->processReferences(Array) (Line: 42)
Symfony\Component\DependencyInjection\Compiler\CheckExceptionOnInvalidReferenceBehaviorPass->processDefinition(Object) (Line: 36)
Symfony\Component\DependencyInjection\Compiler\CheckExceptionOnInvalidReferenceBehaviorPass->process(Object) (Line: 104)
Symfony\Component\DependencyInjection\Compiler\Compiler->compile(Object) (Line: 590)
Symfony\Component\DependencyInjection\ContainerBuilder->compile() (Line: 1284)
Drupal\Core\DrupalKernel->compileContainer() (Line: 873)
Drupal\Core\DrupalKernel->initializeContainer() (Line: 18)
Drupal\Core\Installer\InstallerKernel->initializeContainer() (Line: 465)
Drupal\Core\DrupalKernel->boot() (Line: 414)
install_begin_request(Object, Array) (Line: 114)
install_drupal(Object) (Line: 44)

Additional
Symfony\Component\DependencyInjection\Exception\ServiceNotFoundException: You have requested a non-existent service "theme.manager". in Symfony\Component\DependencyInjection\ContainerBuilder->getDefinition() (line 816 of /var/www/clients/client1/web9/web/vendor/symfony/dependency-injection/ContainerBuilder.php).

Symfony\Component\DependencyInjection\ContainerBuilder->getDefinition('theme.manager') (Line: 456)
Symfony\Component\DependencyInjection\ContainerBuilder->get('theme.manager') (Line: 674)
Drupal::theme() (Line: 22)
_drupal_maintenance_theme() (Line: 710)
drupal_maintenance_theme() (Line: 967)
install_display_output(Array, Array, Array) (Line: 264)
_drupal_log_error(Array, 1) (Line: 570)
_drupal_exception_handler(Object)

 

I will pass along to my betters the message that the install process may break ISPConfig.
Regarding the missing files, these sites are all fresh and new and have zero content on them.
Thank you for the helpful reply.

 

Strange. I have no idea how that would happen-- I guess the one bit of content they all share is that I tried to put a theme on them. I must have done something wrong there. Do you have any pointers on how to browse for what's missing? The real technical wizard here is out for the week but I would like to keep plugging away at it if I can.

I can't get a fresh box to try it on, but to try to eliminate possible bugs I want to do a fresh Drupal install at the very least. You mentioned the basic plan of how to set it up, is this an appropriate install tutorial for ISPConfig3.1 and Drupal8?
https://www.howtoforge.com/drupal_ispconfig

 

Till, you mentioned that that procedure (https://www.howtoforge.com/tutorial/how-to-install-drupal_8-with-apache-and-ssl-on-ubuntu-15-10/ ) could damage ISPConfig. You mentioned that it's a simple process to install Drupal, so I have done that from https://www.chenhuijing.com/blog/drupal-101-getting-started-with-d8/ . What about the option for SSL setup that we get in the first tutorial? Can that process still be done? Thanks!