This happens with pine 4.53 and 4.56, with 4.33 it works. [Can't open .maildir: not a selectable folder] Just to make sure, I renamed the maildir and recreated it with maildirmake .maildir, but it still doesn't work.
pine 4.50 and higher expect an inbox-path starting from $HOME. Instead of ~/.mail/inbox use /.mail/inbox The same holds for incoming-folders and folder-collections.
does this config change address your problem? I have used the latest pine-maildir-4.56 without any problems on a maildir
closing bug as things appear to work in the latest builds just fine.