Auto plugged off?

["Ian D. Leroux" <idleroux@fastmail.fm>]

I'm trying, and failing, to connect to my fastmail account using
IMAP-over-ssl with the following folder specification:

  %inbox:"idleroux@fastmail.fm"/clear@mail.messagingengine.com:993!/

The first time I try to open the folder I obtain the error

  Auto plugged off at mail.messagingengine.com:993
  :elmo-network-initialize-session
  byte-code: Cannot open: elmo-network-initialize-session

Thereafter, it just behaves like an empty folder.
(setq elmo-imap4-debug t) doesn't yield any additional information
(the IMAP debug buffer doesn't show up).

I've checked that I can access the account over unsecured IMAP using

  %inbox:"idleroux@fastmail.fm"@mail.messagingengine.com/

I've also checked that I can log in to the server and list folders
using manual IMAP commands with

  openssl s_client -connect mail.messagingengine.com:993

Access to another of my accounts with IMAP-over-ssl works fine in
Wanderlust.

Does anyone have any suggestions as to how I might obtain more
detailed debugging information about what's going wrong?

Also, assuming I can garner enough understanding of wanderlust's ssl
configuration variables to write a little documentation, is there a
good place to submit it?

Thanks,

Ian Leroux