"Trying to require a method that doesn't exist"

[Bill]

I've used Gnus for a long time, mostly successfully.  Yesterday, I was
sending a long message that I decided I wanted to kill, so I typed C-c
C-c, but it was too late and didn't work.  Emacs seemed to be in a bad
state, and I was in a hurry, so I then killed it off without paying
too much attention to what I did.

Later, i started it again and started Gnus.  Now Gnus won't work; it
gives me the above error message when I try to open any email.  (I can
send email just fine.)

toggle-debug-on-error shows

Debugger entered--Lisp error: (error "Trying to require a method that
doesn't exist")
  signal(error ("Trying to require a method that doesn't exist"))
  error("Trying to require a method that doesn't exist")
  gnus-server-opened((nil ""))
  gnus-activate-group("nnml:mailinglists.reportlab-users" scan)
  gnus-get-unread-articles(3)
  gnus-group-get-new-news(3)
  call-interactively(gnus-group-get-new-news)

I don't know what's so special about that group; I suspect it's just
the one that happened to be being tried when it failed.  I get that
error anytime I try to enter an nnml group.

Any suggestions for things to explore, or do you need more data (and,
if so, what)?

I'm running Emacs 22.2.1 on Debian Lenny, with Gnus 5.11.

Thanks,

Bill