I have installed Grbl 1.1 amd Panel 1.0.9.18. When click Connect button Panel seems to connect to Grbl because I see the version number in the control window. It then pops up this error message. Then locks up.
I can't help with your particular problem, but may I suggest an alternative? If GrblPanel is giving you headaches, try OpenBuilds CONTROL: Download from OpenBuilds Software