The "tag" plugin does not exist The issue is about page loading like the following ones:


Front page
Taxonomy pages (for instance, https://talash.by/taxonomy/term/33)
Any view pages in edit mode /admin/structure/views/view/block_content
Content list page /admin/content
User list page /admin/people


I have a message "The website encountered an unexpected error. Please try again later."

Apache log shows:


  Uncaught PHP Exception Drupal\Component\Plugin\Exception\PluginNotFoundException: "The "tag" plugin does not exist." at /var/www/site/public_html/core/lib/Drupal/Component/Plugin/Discovery/DiscoveryTrait.php line 52, 


The issue happens even if I make a site copy (files and database) on the same physical server, but as different virtual server. Internal pages can be loaded (for example, https://talash.by/terms-of-use) 

I am using Drupal 8.2.4 (standard) on Ubuntu 16.04, with PHP 7.0.8-0ubuntu0.16.04.3 (cli).

These are the non-core modules:


Media                Colorbox (colorbox)                               Module  Not installed  8.x-1.2             


Other                Backup and Migrate (backup_migrate)               Module  Not installed  8.x-4.0-alpha1      
Other                Colorbox Inline (colorbox_inline)                 Module  Not installed  8.x-1.0-rc2         
Other                Disqus (disqus)                                   Module  Enabled        8.x-1.0-rc3         
Other                Exclude node title (exclude_node_title)           Module  Not installed  8.x-1.0-beta1+7-dev 
Other                Ultimate Cron (ultimate_cron)                     Module  Not installed  8.x-2.0-alpha1      
Web services         HAL (hal)
Bootstrap            Bootstrap (bootstrap)                             
Theme   Enabled        8.x-3.1
Bootstrap            Bootstrap Clean Blog (bootstrap_clean_blog)       
Theme   Enabled        8.x-1.0-rc2
Tag is the views cache plugin: \Drupal\views\Plugin\views\cache\Tag, that is in core.

If you are missing that then the files of your installation are messed up, you are possibly missing other files as well. You might want to re-upload all files and also consider using a more reliable deployment method like composer and not upload files over FTP or so.