nnml suddenly not activating groups on startup

[Russ Allbery <eagle@eyrie.org>]

I'm having a very strange issue and am not sure where to look in debugging
it.

I have Gnus configured with nntp as the primary select method and nnml as
the secondary select method, and have a ton of nnml groups.  Due to an
unrelated problem, I've recently had to C-g during a "g" command to check
for new mail and news a few times.  Other than that, no versions or other
configuration have changed.

Suddenly today when I start Gnus most (but not all!) of my nnml groups do
not activate and show in the group buffer with * for the article count.
Pressing enter on a group to read it activates it and then everything is
fine, including the marks.  C-c M-g to activate all groups fixes the
problem.  But when I exit and restart Gnus, or use R, the problem recurs.

Nothing has changed in my startup files and I can't find anything in
newsrc.eld that would be relevant.  Clearly Gnus is storing some state
somewhere, but I have no idea where and can't find it.

Where should I be looking?

-- 
Russ Allbery (eagle@eyrie.org)             <https://www.eyrie.org/~eagle/>