(http://scn.sap.com/thread/1125186) I believe I am having a similar problem.
I have created a varient configuration (rbs99a) to fill out another varient configuration(rb_c_soff). The two varient configurations are in different plants (rbs99a is in Plant "SED", while rb_c_soff is in plant "221"). Rb_c_soff is a non stocked items and has been placed on the BOM as a Item Category of N (Non-Stock Item)
I am running into an error when trying to pull up the varient configuration (rb_c_soff) when it has been selected in the results in CU50 simulation. The error is "message no. CX051. You have selected a BOM item for which characteristic value assignment is not supported. The item is not a configurable object."
Both varient configurations are class type 300. I did not create rb_c_soff and had noticed that it has multiple class assignments. I am typically not use to that.
I had noticed that you suggested using a class type of 200 of the post above. I am not familar with this idea. I am looking for some help to resolve the issue. Any input is helpful. I been stuck on this one for a while.