hehe the rebuild surface option are a bit elaborate.

But I guess you have been told how to do this.

I cannot understand why you get errors.

Can you insert a "usual" isoparm insertion anywhere on the nurbs-surface?

This is how I usually insert an isoparm, the "usual" way;
1) Right klick an object and go into "view-isoparms-mode"
2) Right klick at an isoparm to get a specific isoparm-selection (not-dotted) or klick and drag to get a dotted line (any other isoparm position).
3) Anyway, with a specific isoparm selected (or any position on the surface) you should use the "insert isoparm" tool.
4) And voila.

To insert an inbetween isoparm;
1) Right klick a nurbs-surface and select isoparms from the marking menu
2) Select one isoparm (WHOLE LINE) and hold down the SHIFT key and klick another isoparm (WHOLE LINE) to add another isoparm to your selection (two in total)
3) Now go to the insert isoparms options menu and select inbetween option (I may remember wrong, but I think there is an apply button in the tool-box) and press APPLY.

Remember to shift klick the selection, in order to select more than one "thing".

Other things to know;
To deselect something; hold down Ctrl and select
To add to selection; hold down Ctrl + Shift, to ONLY add.
To add to selection OR/AND invert select; hold down shift and select (I usually use this to add to a selection, but you may accidentally invert select (deselect) something you have already selected.