My Deutsch is a little rust (a lot rusty). What does the error say in English?
I found out ....
How come .... as assembly with out a mirror part ???
I deleted the mirron part ...
and then works the alternative assembly ... !
When using family of parts. Each part must very concise and not assembly driven. No mirrors, inter-part copies, not assembly driven, etc.
If you want FOA/FOP to work well. each part must be in it's simplest form. At least that's what I have found works best. I don't use FOA/FOP much. Was too much hassle.