I've encountered a number of error codes for NotesHTTPRequest.Post(), all of which have the description "user-defined error". I haven't locked down the cause of one, but they are indeed errors of my own cause. But the lack of a helpful description makes it harder to track them down. Maybe it's because the general use case is calling non-Domino HTTP endpoints and the response from Domino is unexpected, maybe it's just that the descriptions of the causes haven't been put in the error handling part.
4844 appears to be because of lack of access to the Domino database I'm calling.
4840 was because I was posting JSON but not setting the content type header.
4846 is another I've had. I am dynamically setting the hostname and internet domain name in IDMA when the server database is opened on the server, and that generally works when called from local replicas. It's possible that there was a problem with the values stored on the occasions it was failing, I'm not sure.
More developer-friendly error messages would be helpful.