is your current user allowed to manage user with the role Manager?
this can be seen in the security of your project :
If this is the cause of the problem, you need to be careful with that option, because allowing that mean that this user role can manage all of the users with the role selected (or change his current user role to the ones selected).
So if this is a security breach in your application you need to find another solution (for example, create a non persistant entity generated when you enter the page with user information that you want to display...).