I get this message when trying to add users to User Roles (i.e. administrators, subscribers, etc.):

Error: User Roles is currently unavailable.
DotNetNuke.Services.Exceptions.ModuleLoadException: String reference not set to an instance of a String. Parameter name: name ---> System.ArgumentNullException: String reference not set to an instance of a String. Parameter name: name at System.Globalization.CultureInfo..ctor(String name, Boolean useUserOverride) at DotNetNuke.Security.Roles.RoleController.SendNotification(UserInfo objUser, RoleInfo objRole, PortalSettings PortalSettings, String Action) at DotNetNuke.Security.Roles.RoleController.AddUserRole(UserInfo objUser, RoleInfo objRole, PortalSettings PortalSettings, DateTime effDate, DateTime expDate, Int32 userId, Boolean notifyUser) at DotNetNuke.Modules.Admin.Security.SecurityRoles.cmdAdd_Click(Object sender, EventArgs e) --- End of inner exception stack trace ---

Any ideas?

I was going to suggest that might be the problem. You'll probably get a similar error if you go to the User Accounts page and try to view all users or that particular user.

Same as my answer before. I suspect that one of the required profile fields isn't getting populated but which one it is I can't tell from the error message.