With the Odoo configuration file set to user workers > 1, PDF creation breaks.

  I get Error: Odoo Warning - Warning: Wkhtmltopdf failed (error code: -11).  

• Webkit installed is wkhtmltopdf 0.12.1 (with patched qt) on Ubuntu 14.04.3 LTS

• lrwxrwxrwx 1 root root 28 May 6 19:31 wkhtmltoimage -> /usr/local/bin/wkhtmltoimage

• lrwxrwxrwx 1 root root 26 May 6 19:30 wkhtmltopdf -> /usr/local/bin/wkhtmltopdf

• PATH=/sbin:/bin:/usr/sbin:/usr/bin:/usr/local/bin for user who is running Odoo service

• Nothing in /var/syslog

• limit_memory_hard = 524288000 in configuration.

What else can I try?