The website encountered an unexpected error. Please try again later (WSOD). Debugging scenario After updating Drupal using drush my website says 'The website encountered an unexpected error. Please try again later'. with white screen(WSOD).

Then when I tried to rebuild-cache using drush there was a drush fatal error originating with rules module. I got it rectified by replacing rules module with 3.x dev version.

I could rebuild the cache and update database using drush. But still the wsod is there

My next move was to enable error reporting. So I added the line

$config['system.logging']['error_level'] = 'verbose';
it did not return any errors on the the screen.
I like to know a debugging scenario for D8 as i faced this same issue for my another D8 installation but that was rectified after rebuilding the cache. I have came across posts by user with similar issues but reasons for their issues were different. So I strongly believe that there should be a debugging scenario that can apply to all.
1- Install Drush if you have not installed it already.

2- Clear cache using drush.

3- Still if you are facing issues start debugging with error logs as following.

Add the following lines to your settings.php

error_reporting(E_ALL);
ini_set('display_errors', TRUE);
ini_set('display_startup_errors', TRUE);


While editing settings.php change its default file permission to writable. 
And you may also add following to your settings.php

$config['system.logging']['error_level'] = 'verbose';


Additional information is available here!

4- If you see any error related to any custom/contributed module after enabling error reporting service.You may overwrite the particular module folder with its latest release or even try with its dev release

5- You can read the server log messages for additional error informations. These log files may vary as per the server configurations. Some information about the server log file available here!