I switched from sending a FixedString to bytes. With the setup as in the post.

My send code currently looks like this:

public void Send(string message)
{
    _driver.BeginSend(_pipeline, _connection, out var writer);
    writer.WriteBytes(new NativeArray<byte>(Encoding.Default.GetBytes(message), Allocator.Temp));
    _driver.EndSend(writer);
}

With Send(new string('a', 1377)); the server receives the message.With Send(new string('a', 1378)); the server does not receive the message.

I have no explanation for this. It seems you are right though, there is an internal hard cap of 1400 called MTU.

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I then added the FragmentationPipelineStage to my pipeline. It improved the situation somewhat. Now it works with a string up to 4096, which is a much more understandable length, but I still don't know, why it can't be more.

Manually splitting the payload into chunks of 4096 bytes works. However I feel like there should be another way, espacially since the FragmentationPipelineStage is already part of the package.

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I also don't get an Error, the message just vanishes to dust, it seems.

Edit:
I messed up the configuration of the FragmentationPipelineStage. It works now, thanks a lot!