Installing Drupal

I would like a particular multi-site installation (I'm testing drupal 8.1.2), as described below.

- many websites with front-end URLs, like:
site1.example.com (website1+DB1)
site2.example.com (website2+DB2)
...
- all the back-end (management) URLs for the above sites as subdirectories of a unique (different) website, like:
admin.example.com/site1 (website1+DB1)
admin.example.com/site2 (website2+DB2)
...

Right now when I create a new Drupal project from

composer create-project --keep-vcs drupal-composer/drupal-project:8.x-dev d8

I end up with a core-install with the

	drupal-composer/drupal-project

scaffold, and the default repository

I would be most grateful for advice on migrating a Drupal 7 multisite to a cloud. I have read "Drupal on Google App Engine and AWS". Here is the current profile:

I have several Drupal 7 sites running on my ISP with no problems. I recently tried to install Drupal 8.1.1 in a test. It installed with no problems, and seems to run OK, except the update module can't get the core update information. Here's the error logs:

Hi,

there is one page I want to upgrade, but to not crash existing one I tried to instal its backup on another, testing domain. Everything went fine but... when I try to enter my testpage I'm autoredirected to oldone so I can't even login to testpage. How can I change it? What file is responsible for it?

thanks,
Mark

[SOLVED--See comment below.]

I'm installing Drupal 8.1.1 8.1.2 on a newly configured CentOS 7 server. FWIW, I have installed a lot of Drupal (though often not done much past that!) and fixed a lot of requirement errors but I haven't seen this one before and I'm baffled.

Here's the text:

Unicode library Error
Multibyte string input conversion in PHP is active and must be disabled. Check the php.ini mbstring.http_input setting. Please refer to the PHP mbstring documentation for more information.