userdb lookup not possible with only userdb prefetch

[Yves Goergen]

Hello,

I am setting up a new server and migrating my Dovecot 1.2.9 config to 
the new 2.2.9 version. There wasn't much to keep from the old version so 
I had to recreate the config from scratch. Now I think I'm done but I 
can't login to IMAP. The log says this:

> dovecot: auth: Error: prefetch(box@****.de,2001:****:7765,<bzAOP5QJXwAgAQpgEAXJAZyOMB1qBXdl>): userdb lookup not possible with only userdb prefetch
> dovecot: auth: Error: userdb(box@****.de,2001:****:7765,<bzAOP5QJXwAgAQpgEAXJAZyOMB1qBXdl>): user not found from userdb prefetch
> dovecot: imap: Error: Authenticated user not found from userdb, auth lookup id=178651137 (client-pid=13711 client-id=1)
> dovecot: imap-login: Internal login failure (pid=13711 id=1) (internal failure, 1 successful auths): user=<box@****.de>, method=PLAIN, rip=2001:****:7765, lip=2a01:****:2, mpid=13717, TLS, session=<bzAOP5QJXwAgAQpgEAXJAZyOMB1qBXdl>

That doesn't help me a lot. Here's parts of my configuration, dumped 
with 'dovecot -n':

> # 2.2.9: /etc/dovecot/dovecot.conf
> # OS: Linux 3.13.0-40-generic x86_64 Ubuntu 14.04.1 LTS
> auth_mechanisms = plain login
> disable_plaintext_auth = no
> namespace inbox {
>   inbox = yes
>   location =
>   mailbox Drafts {
>     special_use = \Drafts
>   }
>   mailbox Junk {
>     special_use = \Junk
>   }
>   mailbox Sent {
>     special_use = \Sent
>   }
>   mailbox "Sent Messages" {
>     special_use = \Sent
>   }
>   mailbox Trash {
>     special_use = \Trash
>   }
>   prefix =
> }
> passdb {
>   args = /etc/dovecot/dovecot-sql.conf.ext
>   driver = sql
> }
> plugin {
>   quota = maildir
> }
> protocols = imap pop3
> userdb {
>   driver = prefetch
> }
> protocol imap {
>   mail_max_userip_connections = 100
>   mail_plugins = quota imap_quota
> }

What's the problem here? How can I resolve this issue?

-- 
Yves Goergen
http://unclassified.de
http://dev.unclassified.de