This warning message indicates that you already have the "provider.user.className" property defined in your Openfire database, and the value you have defined in the openfire.xml file is different than what is in the database. You can remove it from the database via the System Properies page of the Openfire admin console (if you can login), or by deleting the corresponding row directly from the ofProperty table using your DB console, as follows:
delete from ofProperty where name = 'provider.user.className';
