DSfW users can not log into an AD domain via cross forest trust with lower case netbios name

DSfW users can not log into an AD domain via cross forest trust with lower case netbios name.
DSfW users can login only with uppercase while AD users can login with lower and upper case.
DSfW users can login to the DSfW domain with lower and upper case.

Reproduction Steps:

DSFW Domain - dsfw.lan
W2k8R2 Domain - resource.ad.lan
W2k8R2 Domain - master.ad.lan

Trusts are:
dsfw.lan - resource.ad.lan
master.ad.lan - resource.ad.lan

Users from Master and DSFW Domains access the Resource Domain.  If a login (RDP) or doing a 'net use' with an Master Domain User to the Resource Domain, the netbios format works with lower or uppercase letters of the domainname. master\USERNAME and MASTER\USERNAME works.

With a DSFW User to the Resource Domain, only uppercase works. DSFW\USERNAME works while dsfw\USERNAME doesn't work.
Both DSFW Systems are registered in the customers WINS-Servers (example smb.conf [global] wins server = 10.10.10.40).