Not sure if this is the same but found a reason for "G28 Z Forbidden".
While using OctoPrint If I am at home and home Z in OctoPrint I get the "G28 Z Forbidden".
If I first home XY and then home Z in OctoPrint it works fine.
In most cases only use Octoprint mainly for Preheating and video streaming, most of my Print setup functions are done with me in front of the 3D Printers. As at times it still requires the need to babystep Z the 1st layer into that perfect position.
Error "G28 Z Forbidden" is stating that I am already at home when preforming a Home Z.
Computer Programmer / Electronics Technician