First "Knox Tripped" is fixed text - is it 0 or 1? 0 is "not tripped".
Second, only a bootloader error? I'd try flashing it again. If that doesn't work (making sure that you have the F ROM not a J710S or J710V or something else), I'd download it again, from some site that includes the MD5 or SHA<whichever> hash and check the file against the hash first before flashing it. (Free MD5/SHA verification programs are all over the web). If the hash checks (it usually does, but I've had a few downloads [in decades] that have failed, meaning a bad download) then flash it, but if it fails, download the file again.
The worst case is that the bootloader partition itself (hardware) is bad, and that means a new motherboard. But since you just had a ROM in it (the rooted one) that booted, that seems unlikely. A bad flash is more likely. A bad download is less likely than a bad flash, but still more likely than a bad partition.