Gnus archive group processing is broken

[Dave Love]

I have this, taken from the Gnus docs:

  gnus-message-archive-group's value is 
  ((if
       (message-news-p)
       (format-time-string "misc-news-%Y%m"
                         (current-time))
     (format-time-string "misc-mail-%Y%m"
                       (current-time))))

  gnus-message-archive-method's value is (nnfolder "")

Instead of writing to, for instance, ~/Mail/archive/misc-mail-200410,
it sometimes (normally?) writes to ~/Mail/misc-mail-200410.  Then when
Gnus tries to start up subsequently, it fails because it tries to
treat the folder as an nnml group when it's actually nnfolder.

This started happening relatively late in Gnus 5.10 development -- I
don't know exactly when -- with a configuration I've been using for
years.  I reported it to address@hidden without luck, and I couldn't
find my way round the code to try to fix it if I remember correctly.
I think others had same the problem.  It seems serious that Gnus will
fail to start after this happens.