@Steady,
You're spot-on!
Same thing with mine - I use an NEC Multisync II, which has been fine on all of my other Amiga systems, but for some reason, when it's connected during boot-up I can't get past the KS screen, just as you mentioned.
I'm not sure it's a faulty cable, as mine looks fine.
I can reconnect the monitor after KS has booted and everything else works fine...
Maybe it's something to do with grounding? My cable has all of the RGB ground connected together - maybe this is the problem?
Cheers,
Mike.