This is triggered by at least one podcast distributed through Substack. The podcast releases episodes a few days early to paying subscribers, and these subcriber-only episodes appear in the RSS feed.

When castget attempts to download one of them, it receives an HTTP 401 and writes out a 12-byte .mp3 file containing the ascii text {"error":""}, no trailing newline.

The expected behavior in this case is that no file be written at all.

At the top of my castgetrc, I have

[*]
filename=%(date) - %(title).mp3

Which may be involved.