I'm not sure why you need to add UserIdentifier. It's not a parameter of Work-Cover-.Add and isn't mentioned anywhere in the rule (at least in Pega 8.1). Have you traced the call to see where it's failing when you don't include it? Do you have this problem with other activities you call via SafeURL or just Add? When you say the work object isn't created properly, in what way? That might be a helpful clue.