"mailbox is not a directory" message - when switching mailboxes

[cga2000 <cga2000@xxxxxxxxxxxxx>]

I use fetchmail to retrieve messages from a number or POP accounts and
procmail to move the messages to their final destination - mboxes in my
~/mail directory. 

Quite frequently after I run fetchmail "interactively" by hitting the
'G' key, mutt seems to be having problems both with the new mail and my
mailbox menu:

1. The '.' command does not list the mailboxes with new mail. It may
   list nothing at all or only mailboxes that already were already marked
   'N' in the index before I executed fetchmail.

2. The mailboxes with new mail in the mailbox menu are not marked 'N'

3. Trying to access a mailbox causes mutt to display the ".. is not a
   directory" message at the bottom of the screen.. and may cause me to
   be taken to irrelevant areas of my home directory if I insist. Since
   this is a rather confusing situtation I cannot provide an exact
   account of my keyboard actions but they mainly consist in hitting
   the <enter> <c> <j> <k> in succession until I manage to elicit a
   response that is not the "not a directory" message.

I have found a few isolated messages in google that mention this
problem but not much in the way of an explanation/resolution.

What's more curious is that if I am patient the situation usually
rights itself without my doing anything in particular. 

So there seems to be some time-related issue behind this.

Has anyone experienced this.. found what causes it.. and possibly fixed
it?

Thanks,

cga