dovecot - Jan 2013 - users = virual + system (both with ldap backend) => Fatal: setgid(12(mail)) failed with euid=501(...

dear all

probably something trivial, just cannot get my way around 
it, my goal is to get sieve on local-system users too, so 
postfix hands lda over to dovecot
errors with mail for system users:

lda(me): Fatal: setgid(12(mail)) failed with euid=501(me), 
gid=513(Domain Users), egid=513(
Domain Users): Operation not permitted (This binary should 
probably be called with process group set to 12(mail) 
instead of 513(Doma
in Users))

mail storage is in users' homes and these are on nfs3

which bits am I missing, please advise?

regards