I've telnetted to the server to double-check the issue. Here's what I get:
telnet www.livejournal.com 80
Trying 208.93.0.128...
Connected to www.livejournal.com.
Escape character is '^]'.
POST /interface/flat HTTP/1.1
Host: www.livejournal.com
Content-type: application/x-www-form-urlencoded
Content-length: 23
ver=1&mode=getchallenge
HTTP/1.0 200 OK
Date: Tue, 16 Mar 2010 20:16:21 GMT
Server: Apache
Set-Cookie: ljuniq=6en1t8lx8OkEK88:1268770581:pgstat
Content-length: 53
Keep-Alive: timeout=30, max=100
Connection: keep-alive
Content-Type: text/plain
errmsg
Client error: No mode specified.
success
FAIL
As you can see, I do supply the mode-parameter, but for some reason it doesn't get picked up.
What gives?