samba - Nov 2012 - Windows program is verifying owner, group and file permissions

Dear all,

there's UserA and UserB. Their primary group is users. Both are member 
of a group departmentD.

We've a share

drwsrws---	UserA	departmentD	data

that is accessed by a rather unknown business solution software 
(developed with .NET framework).

UserA can write with this business app to this share. UserB can't. When 
I modify the permissions to

drwsrws---	UserB	departmentD	data

it's vice versa.

Both users can read/write this share from Windows Explorer or other 
programs. It's just this .NET application that prevents access. It seems 
like it's checking access rights in a rather strange way.

Is this a known issue? Is there any solution? I set force group to 
+departmentD, but it didn't work. Is it possible to say UserA this 
folder (and all files within) are owned by UserA and UserB it's owned by 
UserB?

Thank you in advance.


Chris