Quoting Adam Bogacki <afb@paradise.net.nz>:
Hmmm... I've read (& shamelessly borrowed) your configs and
commented/uncommented
everything in favour of Maildir in .procmailrc & .muttrc and when, as
user, I try mutt I get
"---Mutt: (no mailbox)
[Msgs:0]---(threads/date)-------------------------(all)---/home/adam/Maildir/
is not a mailbox."
Maildir should be a directory with new, cur, and tmp sub-directories
in it. I can duplicate the error message is I delete the
subdirectories. Are you sure you created them?