Unfortunately my possible workaround in comments 3 & 4 only partially work - cups stops prompting for a password but its smb backend (/usr/lib/x86_64-linux-gnu/samba/smbspool_krb5_wrapper) does not receive the AUTH_UID is not set and therefore cannot spool the job the a Windows print server.
I'm assuming that AUTH_UID can only be set if using a domain socket connection rather than a localhost connection to the cups daemon.
The patch I propose in comment 5 does appear to work.
Unfortunately my possible workaround in comments 3 & 4 only partially work - cups stops prompting for a password but its smb backend (/usr/lib/ x86_64- linux-gnu/ samba/smbspool_ krb5_wrapper) does not receive the AUTH_UID is not set and therefore cannot spool the job the a Windows print server.
I'm assuming that AUTH_UID can only be set if using a domain socket connection rather than a localhost connection to the cups daemon.
The patch I propose in comment 5 does appear to work.