SymptomsPlesk CP log /usr/local/psa/admin/logs/httpsd_error_log contains errors about wrong SQL syntax:
Unable to execute SQL statement (
You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '0''...
ResolutionPlease set 'magic_quotes_runtime' to 'Off' in /etc/php.ini:
magic_quotes_runtime = Off
and restart Apache. This should fix the error.