Drupal 8.x

Heyy 

I'm a student in economy and i have a course called "create value with websites". For this I have to make a imdb-like website but for music in drupal. It's almost done but I have one problem with some views. 

So the main content types of my websites are 

I just updated my drupal website to 8.6.4. I can see my web site. But when I login I get the message " The website encountered an unexpected error. Please try again later. ". I can't get to any of the dashboards including admin to make any changes, run reports, etc.  If I hand type the logout url I can see my website again without the error message. I can't get to the admin panel to change anything. How can I fix this?

Does anyone know why this happens?

I have been running a Drupal 8 site with a dev and production version on a shared hosted server, with a local site on my gentoo box.
I have got some problems with my production site, so decided to clone my local site to the dev site on the shared server (using rsync and mysql for the db) to start with, get that right, then same again to production.
I have got php 7.2 on both.
I am using composer and drush and git.
I know the settings.php file is correct and the .htaccess also
I have cloned this site to a shared server where I have had this working before.

I am interested to know if it is possible to retrieve rendered twig content, js, and css, to be displayed on a non-drupal external site without using an iframe. I have looked into the following options but none seem to work for me:

Implementing the CommandInterface class and using the render method to retrieve the content. This approach seems to require the external site to be drupal as it would require the external site to have access to the drupal ajax commands. I used the following sources for this info.

Hello,

I'm creating a form with four fields .

The first is for 'reference' and when it's content change, make an Ajax callback for inserting text in the other fields issues from database requesting.

The 'reference' Texfield declaration in my FormBase php file :