I found myself wrestling with a server where the Plesk interface suddenly became unavailable without any user intervention. An attempt to start the service was less than fruitful:
[[email protected] ~]# service psa start Key file: /opt/drweb/drweb32.key - Key file not found! A path to a valid license key file does not specified. Plesk authorization failed: HTTP request error  Error: Plesk Software not running. [FAILED]
(Although I included the text from the drweb failure, I later found that it was not related to the issue. However, since it might appear in your logs prior to the HTTP request error, I included it anyways.)
This was a perfectly working server that had no other issues besides this peculiar Plesk issue. Another technician had upgraded the license a few weeks prior, and it was verified at the the time to be working properly. After a bit of Google searching, I found that the solution was to completely stop Plesk and its related services and then start it all up again.
[[email protected] ~]# service psa stopall /usr/local/psa/admin/bin/httpsdctl stop: httpd stopped Stopping Plesk: [ OK ] Stopping named: [ OK ] service psa startStopping MySQL: [ OK ] Stopping : Stopping Courier-IMAP server: Stopping imap [ OK ] Stopping imap-ssl [ OK ] Stopping pop3 [ OK ] Stopping pop3-ssl [ OK ] Stopping postgresql service: [ OK ] Shutting down psa-spamassassin service: [ OK ] Stopping httpd: [ OK ] [[email protected] ~]# service psa start Starting named: [ OK ] Starting MySQL: [ OK ] Starting qmail: [ OK ] Starting Courier-IMAP server: Starting imapd [ OK ] Starting imap-ssl [ OK ] Starting pop3 [ OK ] Starting pop3-ssl [ OK ] Starting postgresql service: [ OK ] Starting psa-spamassassin service: [ OK ] Processing config directory: /usr/local/psa/admin/conf/httpsd.*.include /usr/local/psa/admin/bin/httpsdctl start: httpd started Starting Plesk: [ OK ] Starting up drwebd: [ OK ]
I couldn’t nail down anything within the Plesk log files that would explain the cause of the problem, but this solution corrected the issue instantly.
This issue occurred with Plesk 8.1.1 on Red Hat Enterprise Linux 4 Update 5