SymptomsUpon opening the Plesk Panel "Home" page, you see the "Help" guide in the main frame.
CauseThe root cause of this issue is an incorrect replacement of this file: %plesk_dir%\admin\plib\ui\help.php
The replacement was most likely made in an attempt to fix a security vulnerability, as described in this article: http://kb.sp.parallels.com/en/112303.
According to the above-mention KB article, only these files should be replaced:
To solve the issue, restore the original %plesk_dir%\admin\plib\ui\help.php file.