MySQL Error messages on form creation...

[Deadly]

Hello. I have installed the extension, but get the following error message whenever I try to save a newly created form. Can anyone help me, please?

Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in /homepages/23/d106591793/htdocs/Joomla/libraries/joomla/database/database/mysql.php on line 344

Best regards!