AccessDenied while using UserPrincipal SetPassword method

Question

I am getting an AccessDenied exception while changing user's password using UserPrincipal class.

I am logging to an Asp.Net application through Windows Authentication (domain user). If I try to change other user's password I am getting the access denied exception. If I am doing the same with user that I am logged in, everything is OK - I can change password.

I thought, that if I will get UserPrincipal object using PrincipalContext, I will do anything to which my 'Principal - domain\adminAD' has privilidges in AD.

Body of method:

public bool SetPassword(string userName, string password)
{
    var saved = false;

    var ldapDomainName = "domain";
    var ldapUsersOU = "OU=TEST,DC=domain,DC=com";
    var ldapLogin = "domain\adminAD";
    var ldapPassword = "password";

    try
    {
        using (var principalContext = new PrincipalContext(
           ContextType.Domain,
           ldapDomainName,
           ldapUsersOU,
           ContextOptions.Signing | ContextOptions.Sealing | ContextOptions.Negotiate,
           ldapLogin,
           ldapPassword))
        {
            userPrincipal = UserPrincipal.FindByIdentity(principalContext, IdentityType.SamAccountName, userName);
            userPrincipal.SetPassword(password);
            userPrincipal.Save(principalContext);
            saved = true;
        }
     }
     catch (Exception ex)
     {
        // Log exception
     }

     return saved;
  }

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I have already searched for the answer and I know that there are plenty of answers to similar topics. But none of them explain my problem, I guess.