Hi bushpig,
Apologies for not replying sooner, I tried within an hour of your post but itch.io was experiencing some issues that were preventing me from posting replies. I had to open up a support ticket for them to fix it.
Anyway, PixelCNC asks you to save your project so that the situation which caused the error can be recreated and debugged to determine what caused it. Could you provide step-by-step instructions for re-creating the error? It's imperative that we have exactly the project canvas that are you trying to generate a toolpath from and what tool/operation parameters you're using, etc.. Posting the log file that was produced when the toolpath did not generate correctly would be extremely useful along with your project file that has the canvas and tool definition you were trying to generate a horizontal milling toolpath with.
Though I did happen to find and fix a bug in the horizontal milling toolpath generation algorithm just the day before your post which may resolve the issue. Usually tiny adjustments to operation parameters or CNC/CAM path settings will make an error like this go away. Just nudging your cut step or depth values a tiny amount tends to eliminate the error. Right now however I'm chasing a memory leak in the horizontal milling's algorithm and once that's done I'll be putting v1.37a up for everyone to start testing out.
Charlie