Lightburn has a known bug connecting to Marlin, see link:
My workaround is to open Lightburn, disconnect (change port) leave LightBurn open, then open Repetier Host, Connect to Marlin, once successful close Repetier, return to Lightburn and select correct port and if all is working, it should connect.
I cannot tell you why this works but I saw this in another post in the Lightburn forum and it works for me.