When you open plesk control panel and click on the tab "websites & domains" or FTP accounts (In my case), you got the following error:

Internal error: www/vhosts/domain.com/httpdocs is out of web space
Message www/vhosts/domain.com/httpdocs is out of web space
File Webspace.php
Line XYZ
Type PleskFatalException


This might happen with one specific domain, all others don't show this error.
Although there is plenty of space left in httpdocs and the domain uses very little space, you still receive this error.

Solution :

It could be if "sys_user" has home directory not in the his web space.

Access Plesk database(PSA) and got to "sys_user" table.

The value of column 'home' of your user should be something like 'd:\inetpub\vhosts\domainname\httpdocs'