Hello Sebastian,
I have taken a brief look at the module, and in general it does what you would expect it to.
The parameter I_DARK first of all only works if you combine all operations. So internally it switches to "order combination" instead of "operation combination".
It checks various preconditions, for example whether order contain co-products. In these prechecks I found at least one instance where it throws and error message.
Did you try to catch the error message by adding the implicit exception "error_message = 99" to your function module call?
I am not aware of other BAPIs, except the much "stronger" trim optimization BAPI in APO that is sometimes used for similar purposes.
Regards,
Stefan