Overheating fleet ships can't be salvaged. Only when they're removed from the fleet it works again.
Removing the fleet entirely (delete fleet) doesn't solve it though! You have to remove the overheating member separately from the fleet or you still won't be able to salvage it.
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from T2130 .. also affects new NPC Fleets
Shoot an NPC fleet ship so much that it overheats.
Attempt to salvage it with ship-based Salvage-beams.
Fail miserably.
Add the drifting hulk to one of your fleets.
Attempt to salvage it.
Succeed.