The problem appears to be a bug in the scroller widget code:
Change this line:
var $hdr = $('<table class="' + $tbl.attr('class') + '" cellpadding=0 cellspacing=0><thead>' + $('thead', table[0]).html() + '<thead></table>');
to this (it's just this part $('thead', $tbl).html()
that changed)
var $hdr = $('<table class="' + $tbl.attr('class') + '" cellpadding=0 cellspacing=0><thead>' + $('thead', $tbl).html() + '<thead></table>');
and it fixes the problem.
I've included that correction in this updated demo.
I would also like to suggest you try out my fork of tablesorter which includes lots of improvements, as well as this updated scroller widget (which still has some problems that needs fixing).