It looks like someone tried to both turn the stubborn screw by using a tiny chisel at it's perimeter & also drilling it out.
Don't know about the filler. Hard solder flux cools to 'glass' but that's the only thing that comes quickly to mind. It doesn't really fit in though..
Chisel/pick the hard debris out if the small areas so you don't damage tooling,,use an end mill to take the head off the screw.
You can most likely pull the box down and off the iron then. If not,,you'll have to change endmill cutters and take the screw post out too.
It'll have to go anyway in the end.