Digital output port is always zero

I'm new to DAQ with NI hardware (in my case the NI PCIe-6321 board) and so I tried some simple tasks. However, I fail with writing a "1"/high on a digital output port line. Neither my own C# code (see below) nor NI's MAX work and I can't measure anything but 0 Volts when I connect my multimeter with P0.0 (the pin I want to set to high) and D GND. I tried to disable "Tristate" (I have no clue what it does but I'm sure I don't need that functionality) but it didnt help.

What am I doing wrong? I guess

writer.WriteSingleSampleSingleLine(true, true);

is permanent? Why can't I measure 3.3/5 Volts?

// Reset the device
var dev1 = DaqSystem.Local.LoadDevice("Dev1");
dev1.Reset();


// Create a task
var digitalOutTask = new NationalInstruments.DAQmx.Task();
var digitalOutChannel = digitalOutTask.DOChannels.CreateChannel(
    "Dev1/Port0/line0",
    "",
    ChannelLineGrouping.OneChannelForAllLines);
    digitalOutTask.Control(TaskAction.Verify);

// Write
var writer = new DigitalSingleChannelWriter(digitalOutTask.Stream);
writer.WriteSingleSampleSingleLine(true, false);
digitalOutChannel.Tristate = false;
writer.WriteSingleSampleSingleLine(true, true);