Is triggered by ( ship-visible "Unknown NTF" ), and can be avoided by removing this line.
Players will be unable to target the asteroid, but allied AI ships will still engage the asteroid. Removal of turret-free, and Base Going event trigger from < 30% to < 60% makes this happen much more smoothly.
So all issues reported by OP above that I've also experienced now have fixes available.
Then you can open the FreeSpace Open Installer, which will ask for the GOG FS2 setup file at some point.