Any answer on this? I am having the same problem. For ease of use i will explain my settings below.
My page size is 8.5x5.5 i am imposing it 4up 2x2 on 12x18
in the .fpi file i have stack of 16
when i am in output for the input i choose records 1-16. in the output tabe i have output to multiple files checked with 4 records per file. In theory i should get 1 12x18 sheet per file at this point but i am not.
I am getting same error as the original poster.