jQuery Grid Plugin - jqGrid

I got stuck with my project, due to the fact I can't export the grid-definitions...

When I'm right a export phrase should look like this:

expt=$("#list4").jqGridExport({exptype:'jsonstring',root:'grid',ident:'t'});

the grid #list4 and its navi-bar was created and displayed, now I want to export it.

If I use alert(expt) I get 'null'.
What am I doing wrong?
Firebug does not show me any hints, no errors, the function is defined but it seems like it's not working.

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Short word about my project:

http://handballplaner.de/sandb.....jqgrid.php

is the point where the griddefinitions need to be exported.
Later, when I got this working with your help, my intentions is to offer the opportunity to display all handball-matches (amateurs & professionals) and display them nicely using this great tool.

'handballplaner' will be an open-source project based on Linux, PHP5.3, Lighttpd and CouchDB and of course jQuery and the great tools the community offers - if I get it running one day the way I expect it to work.

Hope someone can help me?
Meanwhile I'll try a little bit more on myself.

Regards,

Steffen