it's meant to do 14.5-32.5khz as input. So not sure that's the problem.
You can input a VGA khz signal..
In the VGA plug that is basically a passthru. ;-)
But as for the RGB inputs that get converted, it doesn't appear to like the higher ranges.
In fact, when you look at the manual, the only 15khz modes is says it works with is for CGA. For RGB in the manual it lists 30.5-32.5.
Of course, we all know that it works with RGB lower at 15khz, but the specs don't specifically say it.
desiv