This might happen if you get too rich, the value of your income variable overflows and causes it to change to a negative value.
This is due to some either poor programming or lack of planning on the game developers' part. As they apparently never expected anyone to get THAT rich. This bug occurs very frequently in all kinds of games.
What I suggest you do is minimize income, don't colonize a billion planets, don't set it so you receive only income, and so on.
In some games programers uses also a negative number. In other words outcome is greater than income. And go below 0 enough and you got your self bankruptcy. Such as SimCity. I havent played Sword Of Stars for a while
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