Help for error on iiwa

Hi, I got this error on kuka iiwa smart pad. I used this repo to connect (https://github.com/epfl-lasa/iiwa_ros) iiwa with ros. Sometime I got "error:ROS disconnected". Then I tried to use "MechanicalZeroPosition" from kuka to move robot back to zero, I got this error and I can do nothing.

Why do I got this error and how do i solve it without restarting robot?

Thanks for any help!