Unable to update PrestaShop content: 500 - Internal Server Error: failed to open stream: Permission denied

Symptoms

• Unable to update PrestaShop content or upload pictures:

  CONFIG_TEXT: 500 - Internal Server Error

• Following error is shown in /var/www/vhosts/system/example.com/logs/error_log file:

  CONFIG_TEXT: mod_fcgid: stderr: PHP Warning:
  file_put_contents(/var/www/vhosts/example.com/httpdocs/modules/gamification/data/data_ES_EUR_ES.json):
  failed to open stream: Permission denied in
  /var/www/vhosts/example.com/httpdocs/modules/gamification/gamification.php
  on line 335, referer:
  http://example.com/admin/index.php?controller=AdminCmsContent&id_cms=4&conf=4&updatecms&token=0&url_preview=1

Cause

Incorrect permissions for website files/directories:

# ls -lha /var/www/vhosts/example.com/httpdocs/modules/ | grep gamification
drwxr-xr-x. 8 apache apache 4,0K ago 24 2017 gamification

Resolution

1. Connect via SSH.

2. Get subscription username changing Domain variable with affected domain name:

   # domain='example.com'; plesk db "select domains.name as 'Domain', sys_users.login as 'Username' from domains, hosting, sys_users where domains.id = hosting.dom_id and hosting.sys_user_id = sys_users.id and domains.name = '$domain'"
   +-------------+---------------+
   | Domain | Username |
   +-------------+---------------+
   | example.com | suexampleuser |
   +-------------+---------------+

3. Fix permissions manually changing suexampleuser with the username of the subscription got in previous step:

   # find /var/www/vhosts/example.com/httpdocs/ -type f -exec chmod 644 {} \;
   # find /var/www/vhosts/example.com/httpdocs/ -type d -exec chmod 755 {} \;
   # chown -R suexampleuser:psacln /var/www/vhosts/example.com/httpdocs/*