Unity Quaternion rotation with rigidbody is influences rotations it shouldn't

Question

Hey fellow programmers,

I have a small problem with Quaternion (who doesn't). I make an airship that is flying using rigidbody. Making it fly forward and turning was not a problem, but now I try to make it lean again the direction of the turn (like a person driving a motor leaning into when doing a turn). The problem is that the ship is making rotations around the x-axis, while i didn't program anything in that would influence it. I checked rotationSpeed, but that only influence y and z-axis. It goes wrong around deltaRotation, where the x-axis is influenced. I tried forcing it to be 0 with deltaRotation.x = 0.0f;, but the ship still goes down without me changing the x-axis. Does anyone know what is happenning, and how to fix it?

if (Input.GetKey(KeyCode.W))
        {
            if (speed < maxSpeed)
            {
                speed += maxSpeed / incrementsSpeed;
            }
        }

        if (Input.GetKey(KeyCode.S))
        {
            if (speed > 0)
            {
                speed -= maxSpeed / incrementsSpeed;
            }
            else
            {
                speed = 0;
            }
        }

        if (Input.GetKey(KeyCode.D))
        {
            if (rotationSpeed.y < maxRotationSpeed)
            {
                rotationSpeed.y += maxRotationSpeed / incrementsRotationSpeed;
            }
        }

        if (Input.GetKey(KeyCode.A))
        {
            if (rotationSpeed.y > -maxRotationSpeed)
            {
                rotationSpeed.y -= maxRotationSpeed / incrementsRotationSpeed;
            }
        }

        forwardPower = transform.forward * speed;

        //Calculate lean value of airship, depending on the speed and turn angle of the ship
        float currentLeaningValue = rotationSpeed.y * speed;
        float maxValue = maxRotationSpeed * maxSpeed;
        float currentValue = ExtensionMethod.Mapz(currentLeaningValue, -maxValue, maxValue, -maxLeaningAngle, maxLeaningAngle);

        //map the value of rotation to inspector rotation
        float realRot = ExtensionMethod.Mapz(transform.eulerAngles.z, 0f, 360f, -180f, 180f);
        realRot = 180 - Mathf.Abs(realRot);
        if (currentValue < 0.0f && realRot > 0)
        {
            realRot *= -1f;
        }

        //Calculate force of leaning
        float leanRotationSpeed = (currentValue - realRot) / incrementsRotationSpeed;

        //Add lean force to turn rotation speed
        rotationSpeed = new Vector3(0, rotationSpeed.y, -leanRotationSpeed);

        //Convert to Quaternion, here something goes wrong
        Quaternion deltaRotation = Quaternion.Euler(rotationSpeed);
        
        //Did this to see if I manually put the x rotation to 0, it would work. Sadly it didn't
        deltaRotation.x = 0.0f;
        
        //Add forces to actual movement of airship
        rb.MoveRotation(rb.rotation * deltaRotation);
        rb.MovePosition(transform.position + forwardPower);