In chat, it will say that a cargo plane is on the way, and then a few seconds later, the green airplane icon on InfoPanel just goes back to gray, like the plane vanished. It does this whenever I try to call in an airdrop, OR when I try to use the Airstrike plugin. What's wrong? Why is cargo planes not working anymore?

Do you have the Arena mod on your server as well?  Or another mod that controls airdrops?