Had my first haven defense where there was a chiron. It ran away eventually. I killed all other pandorans. At that point the Back to Geoscape button wouldn’t work. I alt-tab to end programme. Now the mid-mission saves won’t load. I seem to be able to load the mission start and a previous auto-save on the geoscape just before the haven mission.
Could the chiron trying to escape have anything to do with this? Are the unloadable saves of any use to you?