I successfully got itinerary as
24Aug IADBCN UA992
27Aug BCNFRA LH1139
01Sep FRAIAD LH416
in Y (55000miles + 42560JPY).
Are you sure your routing is IAD-BCN-FRA-IAD AFTER you chose your flights, and itinerary didn't include any non partner code share flights.
Please post your EXACT flights just before booking was refused.