@doguibnu
I suspect the url mentioned is a place to go for help with the error, not a url being accessed during the boot process.
If memory serves (it’s been many years since I’ve done ltsp), after the bios file is loaded via tftp, the initramfs file is loaded via nfs.
First step here, however, would be to go to the client machine, and load a local os (from the hard drive or a flash drive, etc - not via netboot), and attempt to manually load the file in question via tftp. If it loads, then the file exists on the ltsp server and is accessible.