I'm nearly positive the error is caused by not having a correct authorization. I also obtained the same error.
Before the recent change in inventory you could download listings without correct authorization. Now with the inventory it is required. Anyone without authorization can download the non-variation items (which I diid - 142 listings)
Checking for authorization:
On the lower left hand corner of the program there should a key displayed after the name of your shop,he shop name needs to be correct This is the indication of a good authorization.
You can also go to the Check Zety operations and it will check this as well.
Thanks for the info. I will work on providing a clear error message for this along with steps to correct it.
If you have clear authroization and still getting this error let me know.