Are you absolutely sure you are running the same code? Generally i like to copy and file a working program. There may be some advantage to using a source vode management system to corral your G-Code files. Such a system is for the future though
Pictures might help too, im having problems vizualizing what is being discussed here. It is a bit strange that you are only seeing errors on the mirror image part.
The other thing to consider is a controller reboot to go back to defaults. The thought here is that your other part run might have set a parameter that isnt being cleared by the problem program.