A recent update to neomutt is causing a warning message when neomutt is opened if you have used my mutt-wizard to set up your mail. I've already pushed a change that will fix it for new accounts, but you will have to manually make a change for accounts you have already added.
You should be able to fix the problem on installed accounts by running this:
sed -i "s/^mailboxes =\w* =\\+ /mailboxes /" ~/.config/mutt/accounts/*
The issue is that mutt-wizard has historically added "fake" mailboxes to mutt to make the account name appear in the corner of mutt. Due to an update, this is no longer possible. The above line deleted all those fake mailboxes in your config files. GNU utilities required.