When a transfer fails, turn the machine off, then ping its IP address.
If anything responds, then the router has probably assigned that address to another device through DHCP. When LightBurn attempts to send the file to the laser, the other device collides with the laser’s responses.
In that case, you must (somehow) tell the router to not do that, with the details depending which router you have.
A previous discussion covers some of the intricacies: