Network23,
I just wanted to thank you as it seems the original person who posed this question never got back to you on if that worked or not.
I was having the same exact issue with the same exact model type, the HP Elite Book 840. After disabling the MTFTP setting and letting PXE server update we no longer had anymore problems.
Now the machine was only having issues in Legacy only mode which is desied for us as we don't really want to change all Windows 7 images to UEFI accross all locations.
Thanks again guys, love these support forums!
Also, does anyone know any problems with keeping that setting disabled?