Plesk administration panel or websites not accessible: Internal server error 500

Symptoms

Plesk administration panel or websites are not accessible:

    Internal server error 500

The following errors can appear in the  /var/log/httpd/error_log:

    [Sun Dec 13 13:39:23 2015] [error] [client 203.0.113.2] script '/var/www/vhosts/default/htdocs/wp-cron.php' not found or unable to stat
    [Sun Dec 13 13:44:35 2015] [error] [client 203.0.113.2] script '/var/www/vhosts/default/htdocs/wp-cron.php' not found or unable to stat
    [Sun Dec 13 13:47:45 2015] [error] [client 203.0.113.2] script '/var/www/vhosts/default/htdocs/wp-cron.php' not found or unable to stat

Cause

Insufficient disk space:

[root@server ~]# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/md1              4.0G  544M  3.5G  14% /
/dev/mapper/vg00-usr  3.9G  1.9G  1.8G  51% /usr
/dev/mapper/vg00-var  3.9G  3.6G     0 100% /var
/dev/mapper/vg00-home

Resolution

Free up some space on the server.