First off, I see two g-code commands in your file that are not in the Marlin g-code reference: M84, M400. What post processor are you using? The current recommended post processor for Fusion 360 is Don Gamble’s update to the Guffy post processor. See this topic for what a link.
But since you are getting this error in the middle of the file, not at the beginning or the end, I don’t think these g-code commands are at the root of your problem. My two shot-in-the-dark theories are that 1) the post processor is outputting non-printing characters, or 2) there is noise on the line you are using to send the g-code.