NWUsrGrp / NWGroups.Add throws "General API Error" trying to create a group

[blubb...@arcor.de]

Hi folks,
I'm trying to create (and add users to) a group using the NDAP ActiveX
Controls in JScript (WSH), but my script terminates with a "General
API Error" (Code 0).
The relevant lines are:

/////////////////////
var NWUsrGrp = new ActiveXObject('NWUsrgrpLib.NWUsrgrpCtrl.1');
var Context = 'nds:\\\\TREE_NAME\\test';
var ProjGroupName = 'F_0000_IT_TEST_hs';

// next line triggers "General API Error" (Code 0)
ProjGroup = NWUsrGrp.Groups.Add(Context + '\\Projects\\' +
ProjGroupName);

for ( var i = 0; i < Users.length; i++ ) {
ProjGroup.GroupMember.Add(Context + '\\' + Users[i]);
}
ProjGroup.Update();
/////////////////////

eDirectory names are changed but the originals also consist of upper-/
lowercase characters, numbers, underscores and hyphens. The specified
tree and ou do already exist.
I already tried reinstalling the current version of the NDK core stuff
and NDAP ActiveX controls, but to no avail.
The .ocx-files are properly registered, instantiating the NWUsrGrp
object as well as setting and getting trustees and rights with NWVolA
works just fine.