[Dovecot] Quota_rules ignored

[A. Gideonse]

Today I upgraded to version 1.1.16 of Dovecot (and Sieve), because I
wanted to use mailbox quotas. I tried using the instructions on
http://wiki.dovecot.org/Quota/1.1 and simular websites for 4 hours now, 
but it seems that all quota_rules are ignored. Also, whenever I use a 
'*' in my MySQL query I get the error: 'unknown quota backend'. With the 
following query the quota from the database is used (if value is not 
NULL), but the quota_rules are ignored once again:

user_query = SELECT maildir AS home, 5000 AS uid, 5000 AS gid,
CONCAT('maildir:storage=', quota) AS quota FROM mailbox WHERE username =
'%u'

Below is my configuration:

# 1.1.16: /etc/dovecot/dovecot.conf
# OS: Linux 2.6.24-19-virtual i686 Ubuntu 8.04.1 nfs
log_timestamp: %Y-%m-%d %H:%M:%S
protocols: imap imaps pop3 pop3s
ssl_disable: yes
disable_plaintext_auth: no
login_dir: /usr/local/var/run/dovecot/login
login_executable(default): /usr/local/libexec/dovecot/imap-login
login_executable(imap): /usr/local/libexec/dovecot/imap-login
login_executable(pop3): /usr/local/libexec/dovecot/pop3-login
mail_privileged_group: mail
mail_location: maildir:/data/vmail/%d/%n
mail_debug: yes
mmap_disable: yes
dotlock_use_excl: no
mbox_write_locks: fcntl
mail_executable(default): /usr/local/libexec/dovecot/imap
mail_executable(imap): /usr/local/libexec/dovecot/imap
mail_executable(pop3): /usr/local/libexec/dovecot/pop3
mail_plugins(default): quota imap_quota
mail_plugins(imap): quota imap_quota
mail_plugins(pop3): quota
mail_plugin_dir(default): /usr/lib/dovecot/modules/imap
mail_plugin_dir(imap): /usr/lib/dovecot/modules/imap
mail_plugin_dir(pop3): /usr/lib/dovecot/modules/pop3
auth default:
   mechanisms: plain login
   debug: yes
   passdb:
     driver: sql
     args: /etc/dovecot/dovecot-sql.conf
   userdb:
     driver: passwd
   userdb:
     driver: sql
     args: /etc/dovecot/dovecot-sql.conf
   userdb:
     driver: static
     args: uid=5000 gid=5000 home=/home/vmail/%d/%n allow_all_users=yes
   socket:
     type: listen
     client:
       path: /var/spool/postfix/private/auth
       mode: 432
       user: postfix
       group: postfix
     master:
       path: /var/run/dovecot/auth-master
       mode: 384
       user: vmail
plugin:
   quota: maildir:User quota
   quota_rule: *:storage=1G
   quota_rule2: .Trash:storage=100M

Any thoughts on how to solve this problem? Thanks in advance!