Usually it is a Joomla problem. Each version of Joomla comes with a different version of MooTools which are not really compatible, so uddeIM also brings its own MooTools (required for 1.0 and 1.5).
In the backend you will find a setting that allows to control which version of MooTools uddeIM should use (the Joomla one or its own). Usually "Auto" should work fine, nevertheless sometimes other components or templates also include their very own version of MooTools, so it is required to change this setting.
In the FAQ you will find a table which shows some combinations I have tested with the default template for Joomla 1.0, 1.5, 1.6, 1.7, 2.5, and 3.0. Or just try the other settings.
Nevertheless when there is a conflict with a third component or template the setting will not help.
A very rare problem is that not the raw AJAX data is transferred back, but because of a configuration error the complete framework, a http error or other garbage which cannot be interpreted by MooTools and an error occurs. The FAQ also contains an example request for the browser so you can test what Joomla returns when the AJAX request is performed.
uddeIM & uddePF Development
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