This article applies to 123-reg servers.
This article will explain what to do if you Plesk Panel.
Within /var/log/sw-cp-server/error_log it may show something like:
2012-07-05 09:39:09: (mod_fastcgi.c.2613) unexpected end-of-file (perhaps the fastcgi process died): pid: 10809 socket: unix:/usr/local/psa/tmp/sw-engine.sock-0 2012-07-05 09:39:09: (mod_fastcgi.c.3400) response not received, request sent: 2001 on socket: unix:/usr/local/psa/tmp/sw-engine.sock-0 for /admin/index.php?redirect=/subscription/list/, closing connection
This normally happens when you remove Hosting from a domain. To resolve it go into the domain, Websites & Domains tab, click Add next to the domain, select Website hosting, re-add a FTP user/password and click OK.