Thanks Slabbi, excellent fixes, there is still some minor issues though.
The autocomplete seems to work perfectly on the first name, but then I add a comma, and wants to write the second name, and then it doesn't work anymore, it won't autocomplete on more than one name

Yes, the new Autocompleter does only complete the first name. That is not a bug, there is no code that allows multi-completes.

Darned

Hope, someone finds time to modify it to allows multiple user names.

I don't want to create new topic as my problem is with autocompleter too

this is what I get in Chrome debug console:
installed on Joomla 1.5

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.