The package samba-krb-printing starts smbspool as user to be able to get a
kerberos ticket for printing. Recent kerberos/winbind setup switched ticket
caching from directory based store to file based store. Therefore a environment
variable $KRB5CCNAME is set to "FILE:/tmp/krb5cc_[uid]". 
The get_printing_ticket.c clears the environment and this probably would unset
the $KRB5CCNAME, but this is not the problem. Switching user does not set up
the $KRB5CCNAME, only login via winbind sets it according to the setting of
"krb5_ccache_type" in /etc/security/pam_winbind.conf. I had to set it to FILE,
because DIR caused other problems.
So either get_printing_ticket.c has to parse/guess the krb5_ccache_type setting
or pam has to run through winbind on session setup when switching users.
Finally get_printing_ticket.c has to set the $KRB5CCNAME before starting
smbspool.