In
this error is persistent in /var/log/sw-cp-server/error_log
2015/08/18 09:55:57 [crit] 26968#0: *430 connect() to unix:/var/run/sw-engine.sock failed (2: No such file or directory) while connecting to upstream, client: 192.168.0.69, server: , request: "GET / HTTP/1.1", upstream: "fastcgi://unix:/var/run/sw-engine.sock:", host: "192.168.0.124:8443"
/var/log/sw-cp-server/sw-engine.log Nothing special..
[07-Aug-2015 04:03:57] NOTICE: Terminating ...
[07-Aug-2015 04:03:57] NOTICE: exiting, bye-bye!
[07-Aug-2015 04:03:58] NOTICE: fpm is running, pid 10398
[07-Aug-2015 04:03:58] NOTICE: ready to handle connections
[13-Aug-2015 04:10:28] NOTICE: Terminating ...
[13-Aug-2015 04:10:28] NOTICE: exiting, bye-bye!
[13-Aug-2015 04:11:11] NOTICE: fpm is running, pid 26957
[13-Aug-2015 04:11:11] NOTICE: ready to handle connections
[13-Aug-2015 04:11:53] NOTICE: Terminating ...
[13-Aug-2015 04:11:53] NOTICE: exiting, bye-bye!
[13-Aug-2015 04:11:53] NOTICE: fpm is running, pid 27454
[13-Aug-2015 04:11:53] NOTICE: ready to handle connections
[18-Aug-2015 09:40:48] NOTICE: Terminating ...
[18-Aug-2015 09:40:48] NOTICE: exiting, bye-bye!
[18-Aug-2015 09:57:51] NOTICE: fpm is running, pid 3281
[18-Aug-2015 09:57:51] NOTICE: ready to handle connections
/usr/local/psa/admin/logs/panel.log
Here is error backtrack for this error.
[2015-08-18 10:47:46] ERR [panel] Invalid URI: start:
0: UIPointer.php:46
UIPointer:

arse(string 'start')
1: functions.php:1475
wizard_get()
2: Abstract.php:71
Form_Abstract->__construct()
3: preferences.php:212
require(string '/usr/local/psa/admin/htdocs/server/preferences.php')
4: plesk.php:44
[18-Aug-2015 10:47:46 Europe/London] PleskBadURIException: Invalid URI: start
file: /usr/local/psa/admin/plib/UIPointer.php
line: 46
code: 0
trace: #0 /usr/local/psa/admin/plib/functions.php(1475): UIPointer:

arse('start')
#1 /usr/local/psa/admin/plib/Form/Abstract.php(71): wizard_get()
#2 /usr/local/psa/admin/htdocs/server/preferences.php(212): Form_Abstract->__construct()
#3 /usr/local/psa/admin/htdocs/plesk.php(44): require('/usr/local/psa/...')
#4 {main}