I tried the same thing.. Booking AMS-DEN: plenty availability both economy and (non-open-miles) business. However:
-IF I select business-class I get the "technical problem" notification
-if I select economy seats: no problem
Called FB helpdesk:
"Operator: You don't really think that there's any business-class awards seats available??"... What??? This the typical KLM arrogance.
"You can get double-price open-miles tickets". No way I'm going to do that
Me: "Could you check a different departure date?" Helpdesk: "Sir, it is very busy and I cannot check all dates for every customer".. <sigh>
I should book a paid business-class ticket using non-KLM airlines ;-) I give up.
PS. interesting thing I found. Flying to US with stopover in Houston: € 89 tax, with a stopover almost anywhere else: € 229 tax!