After a lot of trying I found out that the UndoOff-Procedure prevents HExtrude from working in VW11!
The following code should produce a polygon and extrude it, but the extrusion doesn't happen:
h := HExtrude(LNewObj, 0, 4);
If I take away the UndoOff, everything works fine. Is this a bug or a feature? At least I found no hints in the documentaton and now I wonder if there are more nasty side effects of the UndoOff procedure. Anyone with similar experience?