I get that error regularly. I think the site just plain doesn't work sometimes. Sometimes that error comes up if it the routing is not logical (i.e. south africa to india via europe) and you have to do segments individually. Sometimes it gives the error if there are no seats available. Sometimes it just gives the error for no reason, then the next day it will work.

I've found that the KVS SkyTeam award search tool is much more reliable, though it sometimes returns the same style of error.