Your problem is with UGS. I have seen EXACTLY this problem. It can take a few seconds for UGS to load a file - if it has problems loading it, there isn’t much error reporting. If you press start before it’s loaded, you get exactly this problem. If you haven’t seen the file name and lines appear in the lower right part of the window (on the bottom status line) wait a bit more for the file to load.
Ok, so I’m letting it load - that solves the first (null) problem. Now, starting from my zero point the z is going to the top and setting an alarm and stops the gcode. I should mention that before that, there is an error which pauses the gcode. In the past (on my other machine) I’ve just continued through the bad code until it starts correctly. I don’t know if it’s the parameters of the HDZ - or if the limit z might be having an issue.
I had a very similar issue when first trying the HDZ with UGS and code from Fusion360.
Turned out, my $28 setting was not correct anymore after the HDZ upgrade, and by default the Fusion360 post inserts code that tell the Z to go “home” (G28 G91 Z0), i.e. the top of the Z rail