I have two virtual fields "ok" and "ok1". In the derivation of "ok1"I have "if(GetValue("KDNR") = blank, SetValue("OK","N"), SetValue("OK","J"))".

That sould bee updated with "refreshform"? The Record is not saved jet.

Yes. 

Depends on the derivation of the Virtual Fields and the order of the fields.

If the virtual fields that are set are after the one that "manipulate" them them derivation of the virtual fields will "trump" your attempts to set them.

If this is not the case, the problem might be case etc. as all GUI functions (including OML) is case sensitive when prism functions (COLUMNS) are not.