What to fill gap where plaster has crumbled inside window pane

I’ve removed the old caulking around some windows and in the top corners of one of them, the paintwork and plaster started crumbling off and now I have a large hole and some gaps. The other window is fine.

What is this usually caused by?

In those areas where there are deep large gaps in the corners, how should I deal with/ fill this?

For those areas where there is around a 2mm wide gap, it seems like the gap goes into the wall about 2cm - is it suitable to caulk straight away or should I fill it first (if so, what with)?

Advice appreciated.

Cheers!