Why is this happening?
When the directprint.io client queries https://directprint.io for the printer list, Z-scaler returns an intermediate HTML page requesting authorization or performing an HTML-JS level redirect. The same page works in the browser because it handles the HTML-level redirect to the JSON API.
The directprint.io client is expecting a JSON response. This causes issues with the directprint.io client, preventing printer lists from downloading.
How do I fix this?
On Z-scaler, create a URL category 'Auth and SSL Bypass' and add directprint.io. This will exempt directprint.io from authentication and SSL inspection.