How to Fix
This error can be fixed by simply retrying the operation. In the case of the image above, that would mean relisting, but if the listing is in the Listing Failed state, you would list it instead. Lister will typically automatically retry listings that fail due to this error, but in case you see one that is still failed with this message, it is safe to retry it manually.
You can retry them from the Product Manager by clicking on the menu icon:
And then clicking on the appropriate action in the menu:
Why This Happens
This is an internal Shopgoodwill error. It's not entirely clear what causes this, but this has been known to happen occasionally when shopgoodwill.com is experiencing a large increase in traffic.