Yes, the air drying unit could help in improving the accuracy and stability of the data. It will remove moisture from the air during the pump process and measurement.
Beside the range, the 2 model (15m and 30m) are identical. The accuracy is same. In terms of component, except the air drying unit, both are identical. Of course, you may like to be careful about the tube installation process. It should not be twisted, too much excess tube, and the tube should not go up and down.
Have you had the chance to verify that the pump is overloaded? Is this reflected in the error message? You can use SDI 12 command to check for error. (See manual for SDI 12 command feedback).
Alternatively, what is LED display error message? 1 flash, 2 flash?