I hope the explanation is understandable. If something is not clear please ask.
Preliminary information (not the bug):
I am organising my ships in A LOT of fleets when playing campaign.
As you will understand, this may favorize the appereance of this bug.
When I attack a world, I win the battle and I capture enemy ships, I observed that the captured ships are always added to my 1st fleet (the most left one, see picture below).
If my 1st fleet (most left one) has at least 1 ship remaining after the battle, no bug will occur.
If my 1st fleet (most left one) is completely destroyed in the battle (no ship left = fleet deleted), the game crashes, because it cannot add the captured ships.
The crash appears when clicking the “OK” button of the “Enemy ships captured” window.
I am able to reproduce it in my current campaign. I made a backup of the campaign to analyse this bug, so I was able to test several fleet configurations.
Version Full 1.55