Why does my admin page appear corrupted after using Backup and Migrate?
If you forget to preserve the file permissions when restoring a backup, you'll run into this. Make sure you use tar's p flag when you extract the backup file. Run drush ws to see watchdog error logs and you'll find permission errors like this:
------ -------------- ------ ---------- ---------------------------------------------------------------------------------------------------------------------
  ID     Date           Type   Severity   Message
 ------ -------------- ------ ---------- ---------------------------------------------------------------------------------------------------------------------
  3436   26/May 18:43   php    Warning    User warning: mkdir(): Permission Denied in Drupal\Component\PhpStorage\FileStorage-&gt;createDirectory() (line 145 of
                                          /home/public/core/lib/Drupal/Component/PhpStorage/FileStorage.php) #0 /h

Fix the file and directory permissions, then rebuild the cache with drush cr and you should see: