I've poked into it a little further, and ... it appears to be content-sensitive. I've tried whittling bits out of the post that fails, little by little, and see if I can identify where the problem is. It'll usually post by the time I get each paragraph (or sometimes all but one paragraph) of the post cut down to the single word "test". There's not a lot of useful output, unfortunately; "requesting challenge", "submitting request", "server error", all within maybe a second. I can post fine throuygh the web interface, including the full exact post (exact in all details) I was trying to post through LogJam.

"And is a puzzlement!"