[Alneyan]

Minesweepers posing as warships? Could be actually: Minesweeper has been known to destroy the productivity of some workers. It isn't a very deadly weapon, though.

All four vessels were equipped with weaponry and supplies, though I don't recall for sure whether they used Direct Fire weapons or Seekers. Of course, Seekers are known to have a few bugs, but Direct Fire is a good deal more likely at turn 50 (when the attack occurred).

For what it's worth, we both concluded it was a bug, and I no longer have the savegame as my harddrive crashed since then.