Well it could be from a number of things.
Is the file the right version eg. did you use a forge tool 3 reference file, which is version 1.15 for vanilla and 1.03 for entrenchment, to version you game is. If its fully up to date it will be 1.18 & 1.041.
Has the file that you modified been entered into the manifest file. Any entity file that you put into mod's folder should be put into the manifest file in your mods folder. (usually doesn't crash Sins though, your changes just won't show up)
If you have the right version and your manefst is up to date, the files are very case sensitive and the numbers you change need to have the same number of decimal places as they did before. eg. if you change.
HullPointRestoreRate
StartValue 1.000000
ValueIncreasePerLevel 0.100000
to
HullPointRestoreRate
StartValue 1.555555
ValueIncreasePerLevel 0.155555
Edit: sorry about the difference in how the top, original set data is all spread out like that its not the way I posted it. It should look like how the bottom altered set is arranged.
it should be OK! But if you accidently put a space or extra number or delete a number position then it will crash. I find this to be the most common cause of the crashes, that have anyway. Just a simple typo.
Note; if you have the "Diplomacy" beta installed it comes with a Developer exe. Using this will point out where you went wrong.