It was pointed out (tx MG) that the maildir++ patch was broken because of a wrong calculated maildir size, and this is causing troubles especially with the new dovecot-2.3. Now that patch was repaired but the maildirs' size remains broken. This can be quickly (actually this may take a long time if done with all (-A) users) restored as follows:
doveadm fetch -A text all > /dev/null
If you don't want to take the trouble to restore your users' maildirs, you can always choose to allow the broken size adding this parameter to 10-mail.conf:
maildir_broken_filename_sizes = yes
