Plesk shows 500 Internal Server Error on Linux: [ERROR]: spawning fcgi failed

Symptoms

Plesk login page shows the 500 Internal Server Error

The following errors can be found in /var/log/sw-cp-server/error.log :

CONFIG_TEXT: (mod_fastcgi.c.1068) the fastcgi-backend /usr/bin/sw-engine-cgi -c /usr/local/psa/admin/conf/php.ini -d auto_prepend_file=auth.php3 -u psaadm failed to start:
(mod_fastcgi.c.1072) child exited with status 1 /usr/bin/sw-engine-cgi -c /usr/local/psa/admin/conf/php.ini -d auto_prepend_file=auth.php3 -u psaadm
(mod_fastcgi.c.1075) If you're trying to run your app as a FastCGI backend, make sure you're using the FastCGI-enabled version.
If this is PHP on Gentoo, add 'fastcgi' to the USE flags.
(mod_fastcgi.c.1171) [ERROR]: spawning fcgi failed.
(mod_fastcgi.c.3644) all handlers for /index.php? on .php are down.

Cause

The wrong permissions and owner were set for the /tmp directory and suexec binaries.

Resolution

Connect to the server via SSH and set valid permissions for /tmp, /usr/sbin/suexec and /usr/lib64/sw-cp-server/sw-suexec:

# chown root:root /tmp
# chmod 1777 /tmp

# chown root:apache /usr/sbin/suexec
# chmod 4510 /usr/sbin/suexec

# chown root:sw-cp-server /usr/lib64/sw-cp-server/sw-suexec
# chmod 4710 /usr/lib64/sw-cp-server/sw-suexec

And restart sw-cp-server then:

# service sw-cp-server restart