SymptomsBlank pages are shown instead of PHP pages hosted on the server.
The errors shown in the domains' error_log-s are related to open_basedir restriction.
When trying to disable the restriction by setting the "open_basedir" variable to a non-path value like "none," the process fails with a "PHP below 5.3 version" error.
Any other attempts to disable the open_basedir restriction fail.
ResolutionThis problem has been fixed in one of the latest Parellels Plesk Panel Microupdates. Make sure your installation is up-to-date.
WorkaroundIf you do not wish to upgrade/update, download the attached Config.php file and place it into the /usr/local/psa/admin/plib/PhpSettings/ folder.
After that try to disable the open_basedir restriction on the domains again. The recommended way is to do it from the control panel on the domain's PHP settings page.