Hi peschehimself,
I installed and tried Required Category Extension; it works correctly.
But it seems that there is some issue with javascript - when I try to post an entry without assigning it to a category, Firebug shows this error
el_name has no properties
if (el_name.substring(0,8)=='field_id') {
It is the line from this script:
//<![CDATA[
form_elements = document.getElementById('entryform').elements;
var id_arr = new Array();
for(i=0; i<form_elements.length; i++) {
el = form_elements[i];
el_name = el.name;
toggle = 0;
if (el_name.substring(0,8)=='field_id') {
This error began to appear when I installed Required category extension and set categories to be required for that particular weblog, so it seems that this error is somehow related to Required category extension.