I suspect you are using the MRBS - Joomla bridge, which is a separate project. I know there were some issues with the Joomla bridge and some versions of Joomla.
If you upgrade to MRBS 1.6.1 you will find that Joomla has now been added to standard MRBS as a fully supported authentication method. Details of how to configure MRBS for Joomla can be found in systemdefaults.inc.php.
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I suspect you are using the MRBS - Joomla bridge, which is a separate project. I know there were some issues with the Joomla bridge and some versions of Joomla.
If you upgrade to MRBS 1.6.1 you will find that Joomla has now been added to standard MRBS as a fully supported authentication method. Details of how to configure MRBS for Joomla can be found in systemdefaults.inc.php.