Common Drupal site migration issues

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Error

The website encountered an unexpected error. Please try again later.

This is the message that may appear quite often during migration/reconfiguration procedure. It is too generic to get useful information about what is going wrong. To get more details, change the line of code in
<drupal_root>/includes/errors.inc
find the line print this message:
print theme('maintenance_page', array('content' => t("The website encountered an unexpected error. Please try again later.")));
and change into:
print theme('maintenance_page', array('content' => t("The website encountered an unexpected error:<br> !message<br> Please try again later.", $error)));

Then much more details will show up to help locating the source of error.

Registry rebuild

The error is very possible caused by module updates which may cause version conflicts between drupal core or PHP/Database environments. Rebuild registry can help keep consistence in API level to fix the site errors.

To rebuild registry, a drupal module called "registry_rebuild" needs to be installed.
Create 'registry_rebuild' directory under
<drupal_root>/sites/all/modules
and download the module
wget https://ftp.drupal.org/files/projects/registry_rebuild-7.x-2.5.tar.gz
decompress and copy registry_rebuild.php to the module directory. Run
php registry_rebuild.php

Drush

Though Drush never works, still document here for future reference.

install drush tool first. Drush 8.x is the latest version that can work with Drupal 7.
Follow the instructions to install Drush.

Run Drush from <Drupal_root>.

Cache clear
Trying clear cache using Drush:
Drush cc all
never succeeds.
To do that manually one have to deal with the database table directly by
truncate all 'cache_ ' tables.

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Database Schemes update

Latest version of Drupal core and modules may have changed database table schemes, and this may cause the Error page appear if the site is still using old version of database imported from old host.
D7.89 core have an extra column('changed') added in user table and this leads to the error page when user log in. Use mysql command line to add this column manually.

alter table users add column changed int after created;
update users set changed = 0;
alter table users modify column changed int NOT NULL;
alter table users alter column changed set default 0;

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Drupal Root directory file permissions

sudo chown -R <owner>:www-data <drupal_root>
sudo find <drupal_root> -type d -exec chmod 2750 {} \+
sudo find <drupal_root> -type f -exec chmod 640 {} \+