Could you check if your volume is full (or almost full) ?
The iterative APIs will create few intermediate files which needs some writable space in volume. And this is not possible when the root volume(/vol/vol0) is full. This could be a reason for the error 'NaAPIFailedException:Unable to create iterator'.
I know this thread is a bit old,yet I'd like to share a possible solution:
connect to your filer c$\etc and look for a folder called .zapi - this folder is most likely full! Rename it to e.g. .zapi_ and then restart your web api session again. It will be recreated and you can work again!
I have sent this answer to another (older) thread as well - but maybe it's better off here