You need to add the windows user who starts the DP GUI to the UserList file on the cell manager (/etc/opt/omni/server/users).
Syntax:
"<DESCRIPTION>" "<USERNAME>" "<AD-DOMAIN_OR_UNIX-GROUP>" <HOSTNAME-OF-GUI-SYSTEM> "<DP-USER-GROUP>"
Syntax example:
"User for GUI access" "ADMINISTRATOR" "PEANUTS" snoopy.eu.hp.net "admin"