maildir subdirs not accessible anymore

[Julien Cubizolles]

I noticed that maildir subdirs don't appear anymore in Gnus. For
instance, the

--8<---------------cut here---------------start------------->8---
[Gmail]/Inbox
--8<---------------cut here---------------end--------------->8---

is absent from the *Gnus Browse Server* list of groups and the [Gmail]
group throws the following:

--8<---------------cut here---------------start------------->8---
error "Couldn’t enter nnmaildir+Gmail:[Gmail]"
--8<---------------cut here---------------end--------------->8---

I used to be able to access such subdirs before with the same setup
(nnmaildir populated by mbsync).I'm using Gnus v5.13.

-- 
Julien Cubizolles