title: Prestashop 1.7 white page
body: <p>I have to migrate my prestashop 1.7 to another sever because the OVH crash.</p>
  <p>but i have blank page in back and front office.
  No error display with debug mode.</p>
  <p>PHP version : 7.1 (apache server with plesk)</p>
  <p>any idea ?</p>
  <p>warning :</p>
  <p>mod_fcgid: stderr: PHP Warning: include(): Failed opening '/var/www/vhosts/xxxxx.be/httpdocs/vendor/composer/../psr/log/Psr/Log/LogLevel.php' for inclusion (include_path='/var/www/vhosts/xxxxx.be/httpdocs/tools/htmlpurifier/standalone:/var/www/vhosts/xxxxx.be/httpdocs/vendor/pear/pear_exception:/var/www/vhosts/xxxxx.be/httpdocs/vendor/pear/console_getopt:/var/www/vhosts/xxxxx.be/httpdocs/vendor/pear/pear-core-minimal/src:/var/www/vhosts/xxxxx.be/httpdocs/vendor/pear/archive_tar:.:/opt/plesk/php/7.1/share/pear') in /var/www/vhosts/xxxxx.be/httpdocs/vendor/composer/ClassLoader.php on line 444</p>
  <p>mod_fcgid: stderr: PHP Fatal error: Class 'Psr\Log\LogLevel' not found in /var/www/vhosts/xxxxx.be/httpdocs/vendor/symfony/symfony/src/Symfony/Component/Debug/Debug.php on line 56</p>
  <p>mod_fcgid: stderr: PHP Warning: include(/var/www/vhosts/xxxxx.be/httpdocs/vendor/composer/../psr/log/Psr/Log/LogLevel.php): failed to open stream: No such file or directory in /var/www/vhosts/xxxxx.be/httpdocs/vendor/composer/ClassLoader.php on line 444</p>