tl;dr

Set the Server External Port (for port forwarded API calls) to your TrueNAS exposed port (not the default 8083).

The fix

If you’re using Calibre Web on TrueNAS to sync your Kobo e-reader, you might run into a frustrating issue: the Kobo device only shows book titles but fails to download books, covers, or metadata, throwing a “Download failed” error.

I followed Jordan Palmer’s guide for the initial setup the connection, but the sync didn’t work properly. The fix was simple: head to Admin > Feature Configuration and update the Server External Port (for port forwarded API calls) to match the port you use to access TrueNAS in your browser (not the default 8083).

Save the changes, resync the Kobo device, and you should be able to download your books without issues.