Plesk (Linux) showing 500 internal server error

There may be a lot of reasons for the internal server error in plesk. Here in my case the situation is as follows.

FIrst of all i stopped 'psa' service and then i started again.

[root@plesk ~]# /etc/init.d/psa stop

[root@plesk ~]# /etc/init.d/psa start
Starting psa... done
Starting xinetd service... done
Starting named service... done
Starting mysqld service... done
Starting postgresql service... not installed
Starting psa-spamassassin service... done
Plesk: Starting Mail Server... already started
Starting psa... done
Starting drwebd service... failed

drwebd service failed here. Then i check the plesk service's error log and found the following entries.

[root@plesk ~]# tail -f /var/log/sw-cp-server/error_log

2013-07-16 12:42:23: (connections.c.1737) SSL (error): 5 -1 0 Success
2013-07-16 12:42:23: (connections.c.1737) SSL (error): 5 -1 0 Success
Cannot find config item ["global/SERVERsocket==:8443", ".php", 0]
2013-07-16 12:42:24: (mod_fastcgi.c.1000) 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:
2013-07-16 12:42:24: (mod_fastcgi.c.1004) 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
2013-07-16 12:42:24: (mod_fastcgi.c.1007) if you try do run PHP as FastCGI backend make sure you use the FastCGI enabled version.
You can find out if it is the right one by executing 'php -v' and it should display '(cgi-fcgi)' in the output, NOT (cgi) NOR (cli)
For more information check
2013-07-16 12:42:24: (mod_fastcgi.c.1012) If this is PHP on Gentoo add fastcgi to the USE flags
2013-07-16 12:42:24: (mod_fastcgi.c.1105) [ERROR]: spawning fcgi failed.


I checked the available diskspace in each disk using 'df -h' command and i found that the disk mounted /tmp directory is full and there is not enough space to create temporary files to run plesk.

I found some files as shown below using the entire space.

[root@plesk ~]# ls -la /tmp
-rw-------   1 psaadm sw-cp-server      200000 Jul 16 11:17 dumpAkwhRE.tar
-rw-------   1 psaadm sw-cp-server   4096 Jul 16 11:18 dumpY2sRjw.tar

I removed one of the 'dump' using more space and restarted the psa service again. Now the plesk service working without any error.