Run this DQL and make sure that 'user_os_name' is null
select * from dm_user where user_name = 'user_name' and user_os_name is not null
If not, set 'user_os_name' field as null from Documentum Administrator or running this DQL.
update dm_user object set user_os_name = '' where user_name = 'wp_admin'
Restart Web Publisher session.