Userdb userdb_mail=, error Mailbox list driver maildir++: maildir_name not supported by this driver

[Marc Roos]

I have this default configuration [0], when use the userdb_mail to point 
to the mdbox location, the inbox subfolders show differently in a 
firebird client. So I decided to copy the whole default mail_location 
into the userdb_mail configuration [2]. But then I get errors [3]. 
Should I change the namespace configuration here, how? My default 
namespaces are inbox and 4archives 

[0]
[@ ]# doveconf | grep mail_loc
mail_location = 
mbox:~/mail:INBOX=/var/spool/mail/%u:CONTROL=~/mail/control:INDEX=/var/d
ovecot/%u/index:LAYOUT=maildir++

[1]
[@dovecot]# cat special-userdb
test:x:8267:231:Account with special settings for 
dovecot:/home/popusers/test:/bin/false:userdb_mail=mdbox:~/mdbox

[2]
[@dovecot]# cat special-userdb
test:x:8267:231:Account with special settings for 
dovecot:/home/popusers/test:/bin/false:userdb_mail=mdbox:~/mdbox:INBOX=/
var/spool/mail/%u:CONTROL=~/mail/control:INDEX=/var/dovecot/%u/index:LAY
OUT=maildir++

[3]
May 13 22:41:32 mail04 dovecot: imap-login: Login: user=<test>, 
method=PLAIN, rip=192.168.10.219, lip=192.168.10.44, mpid=1138, TLS, 
session=<Gk4K8cqIkdHAqArb>
May 13 22:41:32 mail04 dovecot: imap(test): Debug: Loading modules from 
directory: /usr/lib64/dovecot
May 13 22:41:32 mail04 dovecot: imap(test): Debug: Module loaded: 
/usr/lib64/dovecot/lib15_notify_plugin.so
May 13 22:41:32 mail04 dovecot: imap(test): Debug: Module loaded: 
/usr/lib64/dovecot/lib20_listescape_plugin.so
May 13 22:41:32 mail04 dovecot: imap(test): Debug: Added userdb setting: 
mail=mdbox:~/mdbox:INBOX=/var/spool/mail/test:CONTROL=~/mail/control:IND
EX=/var/dovecot/test/index:LAYOUT=maildir++
May 13 22:41:32 mail04 dovecot: imap(test): Debug: Effective uid=8267, 
gid=231, home=/home/popusers/test
May 13 22:41:32 mail04 dovecot: imap(test): Debug: Namespace inbox: 
type=private, prefix=, sep=/, inbox=yes, hidden=no, list=yes, 
subscriptions=yes 
location=mdbox:~/mdbox:INBOX=/var/spool/mail/test:CONTROL=~/mail/control
:INDEX=/var/dovecot/test/index:LAYOUT=maildir++
May 13 22:41:32 mail04 dovecot: imap(test): Error: Namespace '': Mailbox 
list driver maildir++: maildir_name not supported by this driver
May 13 22:41:32 mail04 dovecot: imap(test): Namespace '': Mailbox list 
driver maildir++: maildir_name not supported by this driver in=0 out=378