[Dovecot] Dovecot has no understandable configuration any more

[forum]

Hello,

for many years i used an mailserver with dovecot V1 very satisfied!
http://mailserver.decotrain.de/
One of the reasons to use dovecot was a simply configuration.
Dovecot was the best choice for IMAP for me!

Now i updated the server to Debian testing with dovecot 2.1.7.
It's not possible for me to get a running authentication for virtual 
mail users!

I recently found this mailing and have the same problem:
http://www.dovecot.org/list/dovecot/2012-March/064181.html

The documentation says *nothing* about my problem how to authenticate 
with mysql!
http://wiki2.dovecot.org/VirtualUsers

I can solve the problem only with try and error without a chance in 
this complex system.
It's nice to have all this possibilites but they are not useful if not 
documented.
There should be some clear examples for different scenarios to be found 
in your wiki.


My problem at this time:

I don't know how to configure the userdb to work with mysql 
authentification?
All mail is running under the special user mail.

dovecot.conf
============

disable_plaintext_auth = yes

passdb {
   driver = sql
   # Path for SQL configuration file, see 
example-config/dovecot-sql.conf.ext
   args = /etc/dovecot/dovecot-sql.conf

}

userdb {
   driver = prefetch
}

first_valid_uid = 8
last_valid_uid = 8

first_valid_gid = 8
last_valid_gid = 8

mail_location = maildir:/srv/mail/%d/%n


dovecot-sql.conf
================

driver = mysql

connect = host=/var/run/mysqld/mysqld.sock dbname=... => WORKS!

default_pass_scheme = SHA1

password_query = SELECT password, '/srv/mail/%d/%n' AS home, 'mail' AS 
uid, 'mail' AS gid FROM user WHERE username = '%n' AND domain = '%d' AND 
IMAP_allowed = 'YES'


Errors
======

Apr 30 10:28:22 auth: Debug: client out: OK     1       user=xxxx at yyyy  
     home=/srv/mail/yyyy/xxxx      uid=mail
         gid=mail
Apr 30 10:28:22 auth: Debug: master in: REQUEST 3163291649      5347    
1       87066c907eef8f4e5ed71319b560dd2a
Apr 30 10:28:22 auth: Error: 
prefetch(xxxx at yyyy,::1,<doTKyI/brgAAAAAAAAAAAAAAAAAAAAAB>): userdb 
lookup not possible with only userdb prefetch
Apr 30 10:28:22 auth: Error: 
userdb(xxxx at yyyy,::1,<doTKyI/brgAAAAAAAAAAAAAAAAAAAAAB>): user not found 
from userdb prefetch
Apr 30 10:28:22 auth: Debug: master out: NOTFOUND       3163291649
Apr 30 10:28:22 imap: Error: Authenticated user not found from userdb, 
auth lookup id=3163291649 (client-pid=5347 client-id=1)
Apr 30 10:28:22 imap-login: Info: Internal login failure (pid=5347 
id=1) (internal failure, 1 succesful auths): user=<karsten at dct.mine.nu>, 
method=PLAIN, rip=::1, lip=::1, mpid=5352, secured, 
session=<doTKyI/brgAAAAAAAAAAAAAAAAAAAAAB>


I have no idea how to solve the problem and up to know i could not find 
an example with google.
Please help.

Best regards
Karsten