UserPrincipal SetPassword - The network path was not found

I am trying to change an AD user password using the System.DirectoryServices.AccountManagement namespace. Below is my code, everything seems to be working, I can access the user properties after calling FindByIdentity - but when I try to call SetPassword the following exception is thrown:

Exception has been thrown by the target of an invocation. The network path was not found. (Exception from HRESULT: 0x80070035)

Any ideas? Could this be a permissions issue?

    try
    {
        string sDomain = "domain";
        string sDefaultOU = "defaultOU";
        string sServiceUser = "adminUser";
        string sServicePassword = "password";

        using (var context = new PrincipalContext(ContextType.Domain, sDomain, sDefaultOU, sServiceUser, sServicePassword))
        {
            using (var user = UserPrincipal.FindByIdentity(context, IdentityType.SamAccountName, "accountName"))
            {
                // I can access the user properties
                label.Text = user.EmailAddress;
                label.Text = user.LastPasswordSet.ToString();

                // But setting the password throws the exception
                user.SetPassword("newPassword");
                user.Save();
            }
        }
    }
    catch (Exception ex)
    {

    }

So apparently SetPassword was requiring port 445, whereas simply accessing the user data did not. Allowing this port fixed the issue.