Looks like share permissions (public) are inconsistent with filesystem permissions
Is the AD user in question (accessing from Windows) part of sec-users?
Can you run "adquery group sec-users" and paste the output?
If there's no output or you get (sec-users is not a zone group), then if it's local (/etc/group), then the AD user must be part of that group.
To retest by logging off/back in and trying to map the drive BY short name or FQDN.