EDIT: And OF COURSE I put this in the wrong forum.
lol
Anyways:
My invasion fleet reached Drengi, where I had just cleared out the defenders, both around it and orbiting it. My fleet of transports (two of them) refused to go to the planet's tile, with no "X" symbol signifying they were en-route. I double-checked that there were no defenders, there weren't.
Perhaps the game still recognized defenders as being present? Regardless, I was unable to invade the planet until I reloaded the game, which made the planet vulnerable again.