Magento – Magento 2.3 – Unexpected error Exception: Warning: SessionHandler::read()

I have faced same issue On my shared server.

So what I did is open my env.php file and

find

 array (
  'save' => 'files',
),

and replace with (I created session folder in var directory of my magento)

array (
  'save' => 'files',
  'save_path' => 'var/session',
),

As you already noted, this is a permissions problem. Try the recommended approach to setting permissions, maybe you made some kind of mistake which excluded some files from receiving the appropriate write permissions.

First navigate to your Magento directory on your server:

cd /var/www/magento

Afterwards, try setting the permissions via this command:

find var vendor pub/static pub/media app/etc -type f -exec chmod g+w {} \; && find var vendor pub/static pub/media app/etc -type d -exec chmod g+w {} \; && chmod u+x bin/magento

If that does not work, go use cd /var/www/magento/var/ and type ls -al to see if the permissions on all files are properly set so that your web server can write / delete all files.